Visit to Arecibo Observatory

TRIP TO ARECIBO 

We had an opportunity to spend couple days in Puerto Rico last week and I wanted visit the Arecibo Observatory that is located about 1 1/2 hours drive from San Juan.  I did some research and found out that the visitor center  is open on Sundays.  After reading a very interesting QST article about 432 MHz  EME moon bounce ham radio event I decided to send an email to Angel WP3R  to find out more details on ham radio related activities in the Arecibo ham radio club station KP4AO.   I was positively surprised to get a quick reply from Angel, and even more happy that he was willing to organize a visit for us to see the Arecibo control room  and the antenna system.   After a few email exchanges our visit was confirmed for Sunday March 31st.   We left our hotel in San Juan at 8 AM on Sunday morning  and arrived to Arecibo around 9:30 AM.  Norberto  Despiau  and Dana Whitlow K8YUM greeted us upon our arrival.

ANTENNA 

Dana gave us an excellent introduction to the observatory. He started by explaining the design features of the 1000 ft diameter main reflector. It has spherical shape and is constructed from some 38,778 perforated aluminium panels. These are mounted with +- 2 mm accuracy making this antenna system usable up to 10 GHz frequency band. There is a  moving platform 500 ft above  the main reflector suspended by 18 cables from three towers surrounding the dish. This design enables to track targets +/- 20 degrees off from the zenith.  The two main antenna systems visible on the moving platform are for both transmitting and receiving signals. See Figure 1.

Figure 1. Dana K8YUM explains the antenna systems on the moving platform

The long pole with circular tapered rings is the antenna optimized for 430 MHz band. The white dome contains secondary and tertiary reflectors as well as 6 different antenna systems for various bands between 430 MHz and 10 GHz. These can be swapped in 6 seconds from the control room.

The 18 cables holding this 100 ton platform up in the air are expanding during hot sunny days and contracting during night. To maintain antenna in focus there is an automated system keeping the tension stable. We saw the system as we walked down underneath the main dish. Dana also explained the importance of keeping some vegetation growing under the main dish.   Since the dish is over a natural karst sinkhole it would fill up quickly with mud without vegetation removing the moisture.  Keeping the vegetation under control is another challenge. Cutting weeds on deep slopes of the sinkhole under the main dish is a difficult job.

TRANSMITTER

Arecibo Observatory has capability to provide planetary scale radar imaging.  They have 2  powerful transmitters and combined with very high gain ( 72 dBi at 2.38 Ghz) the effective radiated power can be up to 20 TW at 2.38 GHz.  This power is needed to produce detailed radar images from planets, moons and asteroids. The radar transmission can last for several minutes and is modulated with pseudorandom noise. Signal processing and correlation methods are used to extract data from received echoes. Arecibo can track objects up to Saturn orbit in our solar system, beyond that the echoes come back too late to fit in the observation time window.   

Dana took us to the transmitter room where they had two huge klystron amplifier tubes under repair - black horizontal tube produces 1 MW CW power at 430 MHz and red vertical klystron 1 MW at 2.38 GHz. (see figure 2 below). The grey big box at the center is the 430 MHz transmitter. 

Figure 2.  Transmitter room and klystrons

CONTROL ROOM 

Figure 3 shows the control room where the antenna system is operated. In many cases both research and antenna tracking is done remotely over the Internet. The staff is monitoring the system to ensure safety and address any issues.  One of the biggest problems is radio frequency interference (RFI). Receiver pre-amplifiers are kept cold at 15 K with liquid helium and they are very sensitive so even a tiny RFI source creates a problem. Dana explained that they have a specially equipped van to track down RFI sources, often more than 10 miles away from the observatory. They are also coordinating with air traffic control radar systems during some especially sensitive observations.  

Figure 3.  Marja AB1NL in the control room

RECEIVER SYSTEMS

Received signals are amplified at the antenna platform and brought to the control room either via low loss waveguides or via optical fibers. Figure 4 below shows racks full of advanced receivers and spectrum analyzers. As many principal investigators are operating the radio telescope remotely the digitized signals are available via the Internet. Arecibo has 250 Mbps link which is burstable to 500 Mbps rate as needed. The blue rack on the right side is part of world wide VLBA system  (Very Long Baseline Array). It has large disk arrays that are frequently swapped and shipped to central location for analysis.   

Figure 4. Receiver systems

LOCAL FOLKLORE 

In nearby villages the Arecibo Observatory is called "El Radar".  Locals are convinced that the staff at the observatory are talking to aliens on frequent basis.  In fact  Norberto instructed Dana to show us everything "except the bunkers where they keep the aliens" with a smile on his face.  

Figure 5. Norberto Despiau at control desk

There may be some truth in these beliefs.

Arecibo Observatory was used in SETI (Search for Extra Terrestial Intelligence) project in 1999 to collect data from nearby stars.   It was also used to send the famous Arecibo message (see Fig 6.)  designed by Frank Drake and Carl Sagan.  The message was sent on on 16 November 1974 and it was aimed at the globular star cluster M13 some 25,000 light years away.  

Dana did not confirm or deny  having received any response back yet.  On the other hand he has been working at the observatory only for 7 years.  We may not know the answer before the next 49,960 years.  

I would like to thank Dana, Norberto and Angel  for organizing this very enjoyable visit to the observatory. Special thanks to Dana K8YUM who  spent 3 hours of his Sunday morning with us - very much appreciated.  

73

Mauri  AG1LE   

Figure 6.  Arecibo Message

E51DXX DX-Pedition

LOCATION

Our  DX-Pedition / vacation to Rarotonga was very enjoyable and successful.  We spent almost two weeks on South Cook Islands including a day trip to Aitutaki.  Rarotonga is located in South Pacific between Tonga islands and French Polynesia.  It is in the same time zone as Hawaii (UTC -10h).  Rarotonga is part of South Cook Islands.

We stayed at Sea Change Villas located on the south coast of Rarotonga, almost opposite side to Avarua, the chief town in Rarotonga.

Figure 1.  E51DXX location on Rarotonga

Sea Change Villas has excellent 5 star ratings in Tripadvisor  and after spending there 2 weeks I can assure that the place is just wonderful. When booking the villa back in December 2012 I was corresponding via email to Christina and Deb.  I asked if it was possibile to setup a ham radio antenna.  They responded that if I don't interfere with the Wifi internet service I am welcome to setup my antenna.   It turned out to be a very ham friendly location and I got even some help to pull the coaxial cable. 

There are multiple villas on this well maintained property, including two that are right at the beach (Fig 2.).  We opted for unit #7 that is approximately 150 meters from the beach on a sloping terrain. 

Figure 2.  Sea Change Villas

We arrived to Rarotonga with a direct Air New Zealand flight from Los Angeles around 6:30 AM local time on April 1st.  Upon arrival the warm southern pacific air smelled really good and there was a nice breeze coming from the ocean.   We had a driver waiting for us and he gave us white flower leis and took our luggage.  On the way to the villas James told us about Rarotonga. He explained that the main road Ara Tapu goes along the coast around the island and is about 32 km. There are clockwise and counterclockwise bus routes that go around this road.  There are also plenty of places to rent a scooter,  a bicycle or a car.  We passed by many hotels on the west coast as well as several shops and restaurants.  In about 20 mins we arrived to Sea Change Villas and James helped to unload our luggage. 

Figure 3. Sea Change Villas 

We were positively surprised of the villa - it was very spacious and very well maintained.  We had a nice view to the ocean as well as our own private pool backside. We also found that the refrigerator was well stocked with food so I prepared some eggs and toast for breakfast.  There was also a letter on the kitchen table explaining that the reception opens at 9:30 AM.  

After settling in  and exchanging more comfortable clothes I walked down to beach to check water temperature.  The morning mist was still visible around the mountains as visible in Fig 3. 

During the two weeks we stayed in Rarotonga the weather was really nice and temperature was around 30 C  (day time) to 23 C (night time).   

When the office opened in the morning John came to provide us orientation to Rarotonga.  We discussed about various activities and locations that we wanted to experience.  John was very knowledgeable and helped us to rent a car  and get our first scuba diving scheduled.   He also provided a really good overview of restaurants, beaches,  shopping and other activities we were interested.  

E51DXX STATION DETAILS 

John also helped me to setup the Buddipole antenna next to the pool and to pull the coaxial cable inside. The telecom office was closed on April 1st as it was a public holiday in Rarotonga but I did erect the antenna to listen the HF bands in the afternoon and evening.

Figure 4. Buddipole antenna

I had two radios with me  - Elecraft KX3  and  Flex-3000.

I had also Powerwerx SS-30DV portable 30 Amp power supply with me.  I had also two adapters to plugin to local 230V AC but I realized later that they were lacking the grounding pin.

It took me a while to assemble the station and connect to Flex-3000.  My trusty old Thinkpad T60 laptop had latest and greatest PowerSDR software that I did test before our departure to Rarotonga.

I had also an Array Solutions AIM UHF antenna analyzer to help tuning the Buddipole antenna to different bands.  I  tested the Buddipole in different configurations such as

• horizontal dipole
• vertical dipole
• L-shaped dipole 

I had a portable mast to erect the antenna up to 18 feet. I had also guy wires to keep the antenna straight.

First day was filled with other activities as well.  I went to get a rental car, we visited the scuba diving place to schedule some dives  and  finally took our snorkling gear to explore the blue lagoon a bit closer.


In the evening I was listening the ham bands  - 20m was wide open and I heard many VK and ZL stations as well as North American stations.  I also heard a few European stations later in the evening via long path.  The sunset was around 6:40 PM local time and eventually the sky turned pitch black.  I went out a few times to adjust the antenna and I was amazed to see Milky Way and Southern Cross. With almost zero light pollution in Rarotonga the sky was really black and I saw many unfamiliar constellations.

Figure 5. E51DXX license

Next morning we headed with my wife Marja (AB1NL) to Avarua  - the small town on the northern coast of Rarotonga. I got my Cook Islands drivers licence at the Police station - a dark gray building next to a gas station.  They also instructed me to go the Telecom office to pickup the E51DXX ham licence.  Telecom has a nice modern air conditioned office  and I asked for Mr. Katoa Banaba to whom I had emailed copy of my passport and U.S. ham licence in advance.  He had prepared my licence  so I walked out of the office in about 15 minutes with a fresh new E51DXX amateur licence in my hand.

We bought some groceries from CITC supermarket near the airport and headed back to the villa.  There are many smaller stores around the island but CITC store is well stocked and had everything we needed. There is also locally grown fruits and vegetables.

After dinner I opened the radio and starting working QSOs.  I was struggling a bit as the station setup was not very stable initially.  I got some signal reports where NZ  hams indicated that I might have some RF in the shack. I had some extra ferrite cores with me so I re-arranged the cables.  I also realized that the missing grounding pin on the AC adapter might be part of the problem. However,  I was able to fix most of the issues and adjust the settings in Flex-3000 / PowerSDR to eliminate the problem and I started getting better quality reports.

As discussed earlier I also experimented with different Buddipole configurations.  It was very easy to bring the antenna down,  adjust settings, measure with AIM UHF analyzer  and lift the antenna up again. As I got better feel of the bands and different antenna configurations I settled with the L-shaped configuration (see Figure 6. below) that seemed to work extremely well.  I used this setup both at the beach near water front as well as at the villa. Chris W6HFP and Tom K2GSJ have a neat Youtube video that explain how to do the antenna setup.

In the following days there were several great 28 Mhz openings  so I took my Elecraft KX3  and set the antenna right at the beach.  I made many great SSB contacts to Europe,  USA and Japan with only 5 Watts.  Many hams were amazed how well they heard my QRP station.  In many cases they had multielement Yagi and 1 kW amplifier on their side, and they gave me 56 to 59 signal reports. KX3 has excellent receiver and I did hear more stations than I could work with.

Figure 6.  Buddipole in L-shaped configuration.

I needed some power adapters  and I was wondering if there is anything available in such a remote South Pacific island.  I called Jim E51JD who we had already met at lunch in Avarua. There is a fairly new electronics store near the airport called JayCar.  I visited the store and talked to Denny who is the store manager.  He was very helpful  and not only had exactly what I needed but he also helped to test that everything worked. See figure 6 below - he is testing my Powerpole connections.

The store itself is like a very well equipped RadioShack. He had everything from connectors, batteries, solar panels to actual electronics components.  Denny mentioned that even the telecom engineers visiting from New Zealand find some components they need as they are installing a new 3G network on the island. I did visit the store couple times  during this DX-Pedition. I wish my local RadioShack store near Lexington would be as well equipped as this one.    

Figure 7.  Denny  from Jaycar is helping to fix a new power adapter. 

SCUBA DIVING 

We did multiple scuba diving trips during this vacation.  We used Pacific Divers - their office is located on the east side of the island.   

Figure 8. Marja returning from a dive

Figure 9.  From left: Alex, dive master Hayley, Marja, Mauri and Mitch

 Most of the trips started from Avarua harbor as the wind was blowing mostly from south west during the first week.  The conditions were good - sea was calm and visibility was  around 30 meters (100 feet).

We had scuba diving scheduled typically in the morning and we explored the SS Mai Tai shipwreck near the Avarua harbor.  We also visited Edna's achor on this dive trip.  During the following 12 days we visited few other dive sites around the island.

Coral Gardens was an impressive dive site and I did capture on video some strange creature that might have been a big octopus hiding under a big rock.

We also learned some new tricks.  Diving backwards over the side of the boat was a new experience for me. The first time felt pretty awkward but it was really the only practical way to get into water with air tank and all the other gear as the boat was pretty tightly packed as you can see from Figure 8.

Figure 10.  Marja coming to surface

Unfortunately I did have some trouble equalizing my ears so after my 4th dive trip I had blood in my mask and had to stop and focus on snorkeling. My both ears were pretty clogged for the rest of the vacation.   However,  I did have a lot of fun and got to play with my new GoPro2 camera.  I still need to edit all those underwater video clips - I have literally Gigabytes of video and pictures.

Scuba diving in Rarotonga was a lot of fun but I also enjoyed snorkeling.  For example in the place called "Fruits of Rarotonga" there is a lot of tropical fish and corals if you swim past the big red buoy.

On our day trip to Aitutaki we had also opportunity to snorkel in the lagoon. What a fantastic place - the natural beauty of this lagoon must be experienced - photographs won't do it.  We finished our lagoon journey to One Foot Island (Tapuaetai) where we had lunch and got our passports stamped.
Aitutaki has also been used as the location for US TV program Survivor:Cook Islands.    

SOCIAL ACTIVITIES 

On Friday April 12th I had also a very nice lunch with Jim, E51JD and Here (friend of Barb, N1ICQ) at Tamarind restaurant. I learned quite a lot about the island, its history and the people.  Jim has been living in Rarotonga for 40 years and Here is native resident so they were both very knowledgeable. The island has changed over these years,  quality of housing has improved and tourism has become the biggest source of income. Many people have also moved to New Zealand and Australia, especially from more remote islands of Cook Islands group.  In Rarotonga there is about 10,000 residents and on annual basis they have some 100,000 tourists visiting these islands. When the new airport was opened in 1974 it brought many new visitors to these southern pacific jewels. 

Figure 10. Lunch with Jim E51JD and Here.

We did also some antenna tuning at Jim's place.  He has an impressive station on the north east corner of Rarotonga with antennas covering almost every band. I heard Jim E51JD multiple times on the air during my visit to Rarotonga even though I was on the other side of Rarotonga mountains. His 30m/17m/12m vertical antenna was a little bit out of resonance so we took it down and adjusted the upper element.  While we had the antenna analyzer available I measured all his other antennas as well and they got a perfect score.  Jim is fairly close to the beach and he explained that winds are bringing salt everywhere so corrosion is a big problem.

WORKING DX STATIONS
As this DX-Pedition was really a vacation I was not too worried about working around the clock or trying to maximize number of contacts. I worked at a leisure pace and did not spend all nights chasing for DX stations.  However,  I did have a couple good pileups  and I am very grateful for the patience of fellow hams who were calling me.  In several cases I worked a station and they gave up their frequency and put me on the DX cluster.  This did help a lot as I spent quite some time calling CQ  without success as my signal was pretty weak in Europe and USA. Once I was spotted on the DX cluster a big pile up developed and I could work over 100 stations in a short period of time.

It was also  interesting to hear how the band conditions changed pretty quickly. After sunset in Rarotonga I could work stations around the Pacific rim. I also participated the ANZA net almost every day  at 14.183 Mhz at 05:15 UTC.  I got many good contacts through the net.

Figure 11. E51DXX working a pile up 

LOG SUMMARY 

As discussed earlier many of these contacts were made with only 5W with Elecraft KX3  and L-shaped vertical dipole near the beach.  As many hams have discovered earlier a vertical dipole near large body of salt water is an excellent antenna for DX work.  In my case I tested various Buddipole configurations and finally decided to stick with L-shaped vertical dipole (see figure 6. above). It was very easy to tune to accurately to resonance, it provided really good signals on reception and I got great signal reports even from stations 10,000 miles away.  I was literally blown away how well this QRP setup (see Fig 12. below) worked.

Figure 12. Elecraft KX3 - 5 Watts and vertical dipole at the beach

With Flex-3000 my transmit power was around 50-70 W. As the villa was about 150 meters from the beach I used the Buddipole antenna at maximum height of 18 feet and carefully tuned it to resonance. I did notice about 2 S units difference on several stations compared to reports by Jim E51JD - he was located on the north side of the island. Jim was  using a 3 element Yagi  that may explain part of the difference. I also worked Jim a few times - he was only 5 - 6 km away but we had Te Manga,  a 652 meter (2139 ft) high mountain between us. I am not exactly sure how and where the radio waves reflected but they certainly did not pass the solid rock between us.

Total number of QSOs  was 776 and they are mostly on 20m and 10m bands. I did spend time also on 15m and 17m bands but to my disappointment I did not hear much activity and got only a handful of contacts on these bands. 10m band openings were totally unexpected and a great experience.

10M	362
15M	5
17M	7
20M	402
TOTAL	776

I was positively surprised how well my simple station performed. I worked total of 49 DXCC countries listed below. There was a couple new ones for me like Samoa and Fiji. I was also very happy to get 10 stations from Finland in my log, as the distance is over 9700 miles and my VOACAP propagation prediction did give only a slim chance to make these contacts with QRP power levels.  Out of blue also Nick RI1ANP called me from Antartica on 10m band.

ALASKA	1		COLOMBIA	1		HAWAII	11		NETHERLANDS	6		SERBIA	1
ANTARCTICA	1		CROATIA	1		HONDURAS	1		NEW ZEALAND	18		SLOVAK REP.	3
ARGENTINA	2		CZECH REP.	3		HUNGARY	1		NORWAY	1		SOUTH KOREA	1
AUSTRALIA	74		DENMARK	3		INDIA	2		PAPUA NEW GUINEA	1		SPAIN	14
AUSTRIA	4		ENGLAND	5		ITALY	18		POLAND	4		SWEDEN	4
AZORES	1		FIJI	2		JAPAN	80		PUERTO RICO	2		SWITZERLAND	3
BELGIUM	2		FINLAND	10		LITHUANIA	3		RUSSIA (ASIATIC)	2		TRINIDAD AND TOBAGO	1
BRAZIL	12		FRANCE	9		MADEIRA ISL.	1		RUSSIA (EUROPEAN)	1		UKRAINE	1
CANADA	5		FRENCH POLYNESIA	1		MARTINIQUE	1		SAMOA	2		USA	171
CANARY ISL.	3		GERMANY	20		MEXICO	2		SCOTLAND	1

I have uploaded my log to eQSL.CC  and to LoTW.  I already had a stack of QSLs waiting in my mail when I returned home from this trip. Creating a printed QSL card  and sending  them out will take few months so please be patient with me.

Update May 11, 2013:  QSL cards arrived (see Fig 13. below) and I sent the first batch to those who send me QSLs directly.

Figure 13.  E51DXX QSL card front

73
Mauri  E51DXX  /  AG1LE 

Antenna Experiments - Human Body Resonance Frequencies

Inspired by "Ionic Fluid Antenna" article by  N9ZRT David Hatch I started wondering  whether  human body could be used as an antenna.  

Using human body as an antenna would have multiple obvious benefits such as 

Human body by Leonardo da Vinci

• mobility  -  ham radio operator would have the antenna always with him / her
• low cost  -  no need to buy expensive materials 
• small size -  no additional wires or aluminum tubes to carry with you 

However, there are also a lot of risks involved so I don't recommend experiments as you may hurt yourself by causing RF burns.

It is a well known fact that human body contains ionic fluids and it both absorbs and emits electromagnetic signals.   In medical field these human body properties are used for many different purposes such as diagnostic imaging or diathermy treatment.  

For example MRI (magnetic resonance imaging) scanners  send and receive RF signals at Larmor frequency.  The frequency range depends on  magnetic field strength of the scanner  -  proton  ( ¹H ) resonance frequency is 63 Mhz  at 1.5 Tesla field strength typically used in MRI scanners designed for hospitals.

RF coil for body imaging

In early part of my career I worked  in medical imaging field and thus have some hands on experience working with MRI  RF coils.  Designing RF coils for different body parts is a challenging job.   In order  to obtain best possible signal-to-noise ratio the RF coils need to be close to the target. On the other hand RF field homogeneity is also important to create clinically acceptable MRI images.  Human body couples  with  MRI RF coils so calibration and impedance tuning is important and is typically quick automated operation - many patients have difficult time staying still in the magnet for extended periods. Also,  SAR (specific absorption rate) level  (the RF energy absorbed by human body)  must to comply with FDA regulations.  

However,  most hams don't have superconductive 1.5 Tesla magnets or MRI RF coils in their ham schacks so I wanted to explore some other means how to couple RF energy to human body,  understand at what frequencies it would be resonant and how good of an antenna human body would be.        

After reading several articles on this topic it became clear to me that  human body resonates at multiple frequencies  and these frequencies also depend  on the environment. Human in free space  vs.  standing on the ground would have different RF characteristics like any normally used antenna. 

WARNING:  RF Safety

I wanted to study this a bit further  and  do some measurements myself.  I determined that by using the AIM UHF antenna analyzer (created by W5BIG Robert Clunn) I could safely measure my own resonance frequency. The power output of this excellent device is between 0.2 - 20 microwatts that is well below  SAR limits (for mobile devices typically 2W / kg for whole body).  Safety is obviously utmost important as I did not want to get RF burns or cause other tissue damage due to this experiment. 

Coupling a coax transmission line to my body was the next problem.  I created  a simple coupling device by attaching two electrodes designed for a transcutaneous electrical nerve stimulation (TENS) machine to a 50 Ohm coaxial cable.  The distance between electrodes was 16 inches and the coaxial feed point was in the middle.This distance allowed me to test different configurations in vertical and horizontal modes. Picture below shows the RF feeder device.

RF feeder

I analyzed first this RF feeder circuit itself. Hanging the feeder circuit in vertical position inside my ham shack I measured  SWR and complex impedance magnitude over  1 - 999 Mhz frequency range.  This gave me a baseline to compare how attaching the electrodes to my skin would change the resonance frequencies. The SWR was over 2.0 between 1 and 244 Mhz. The dip in SWR was centered around 252 Mhz corresponding to 46.8 inches wavelength. Given that the coupling circuit is essentially a half wave dipole with 16 inch length the velocity factor of TENS electrode material  is 16 / 23.4  = 0.68.  The SWR also dips below 2.0 around 860 Mhz.

RF feeder  measurement

The next step was to attach the electrodes on my skin while in supine position and run the same test again. 

I changed the SWR scale to get more detailed view.  This time there are 2 main dips in 65 Mhz (181.4 inches wavelength) and 112 Mhz (105.5 inches wavelength)  frequencies. Also, in 600 Mhz range the SWR dips below 2.0.  The prominent dip at 112 Mhz corresponds to 105.5 inches wavelength

Human body resonance

This result corresponds to literature.  Human-body resonances fall typically in the frequency range of 30 - 300 Mhz.

Looking at the SWR graph above  50 MHz ham band represents roughly 1:3 SWR ratio from impedance matching perspective.  I also noticed that the graph changed  as I moved my arms and legs.  With some practice  it is possible to shift the SWR minimum by about 2 MHz just by moving self.

The posting I did in eham.net has created a lot of discussion mostly focused on safety risks and RF burns. I want to emphasize that RF safety is utmost important and human body should not be exposed to RF signals.  Even at small power levels RF can cause diathermic heating in tissues causing painful tissue damage.

The fact the human body resonates at multiple frequencies presents also a problem if one is exposed to strong RF fields (like standing in front of directional antenna with RF transmission on).  The RF radiation may cause "hotspots" where induced currents are causing localized RF burns, even though the overall SAR limit is not exceeded.

Ionic fluid antennas may have some special applications but human body should not be used as an antenna.

So - it looks like traditional metal objects are far superior as antennas if the purpose is to effectively radiate the RF power and not to use it to generate heat.



73   Mauri  AG1LE

Antenna experiment - Delta Loop for 7 Mhz band

I  had a 80 m dipole that was not performing too well because it was only at 25 ft  height due to physical constraints in my backyard.

Looking at my options I decided to re-use the antenna materials and make a delta loop for 7 Mhz band.


Using Kok Chen's (W7AY) CocoaNEC software  I created a NEC2 model for Delta Loop.
Using a programming language resembling C it is really easy to create these models  - see Fig 2. and code sample below:


model("deltaloop - corner feed ")
{
real h, dh, fr, l, vf;
element _e1, _e2, _e3;
 
vf = 0.95;      //velocity factor of wire 
fr = 7.1;       //frequency in Mhz
l = c/fr/vf;     // length l = 44.44662 m @ 7.1 Mhz
h = 5;         // height of the bottom h = 5m, apex at h+dh = 17.8m

dh = sqrt((l/3*l/3) - (l/6)*(l/6)); 

// three elements are each L / 3 length - using #14 AWG wire // and 21 segments for each element in the model
_e1 = wire(0, -l/6,  h, 0, l/6, h, #14,  21) ;  
_e2 = wire(0, -l/6,  h, 0, 0, h+dh, #14, 21) ;
_e3 = wire(0,  l/6,  h, 0, 0, h+dh, #14, 21) ;

voltageFeedAtSegment(_e3,1.000000,0.000000,1);
averageGround();
frequencySweep(6.9, 7.3, 40);
}

Fig1. Delta Loop Current Magnitude 

Fig2.  Model dimensions

Fig 3. SWR plot of NEC2 model

Fig 4.  3D shape of radiation pattern

The NEC2 model above is designed for  SWR minimum  at 7.1 Mhz. 

Fig 3. shows that Zmag at resonance is about 140 ohms making SWR at the minimum about 2.8 : 1  without any impedance matching.

By changing model parameters like feed point, height and ground quality you can influence the impedance.

In my back yard I have about 70 ft tall tree so having the loop apex pulled up at  ~ 60 ft is doable with a proper support rope going through the top branches of the tree. 

The model also shows the 3D radiation pattern that has a weird looking indentation on one side. This is also visible on the azimuth graph Fig 5.  below.

Fig 5. Azimuth      and      Elevation

The  antenna feed point (Delta-C Center Insulator) is at the bottom loop corner  very close to the house - see the picture below. I added two ferrite chokes to suppress common mode current and  also put some new self fusing silicone coax tape around the PL239 connector to prevent moisture entering the coaxial cable.  

Fig 6.  Antenna feed point

Fig 7.  The apex of the delta loop

The apex is barely visible in the Fig 7.  Following the black support rope from left bottom corner (from apex of my 18 Mhz delta loop) towards middle of the photo you can see a gray insulator in the middle of the branches.

Fig8.  The 7 Mhz delta loop - 145 feet total wire length

I wanted to get the resonance frequency at 7.1 Mhz.  After doing some measurements using AIMuhf antenna analyzer I  added 11 ft of wire to the existing 80 m half wave dipole to convert it to a full wavelength 7.1 Mhz loop antenna.

Original 80 m antenna was 134 feet, so the new length is 145 feet (44.196 meters). This corresponds to the NEC2 model fairly closely  (NEC2 model L = 44.44 meters).

From the delta loop articles in the internet  you can get all kind of formulas such as  L = 1005/f  for the wire length. 

As the local environment influences the antenna it is better to  make the wire a bit longer and use an antenna analyzer to cut it to the resonance length. 

The SWR minimum 1.33:1 was at 7.1 Mhz  after adjusting the wire length.  SWR is below 2.0 between 7.0 and 7.2 Mhz as demonstrated by the red line in Fig 9. below. To validate AIMUhf results I measured  SWR and it was around 1.5 also according to my other SWR meters. 

From the NEC2 model  I expected that I would need a 2:1 balun or some other impedance matching network as the model did show 140 ohm impedance at the feed point.

However, the measured Zmag (green line) is  37.5 Ohm at  7.1 Mhz resonance frequency. This measurement was not done at the feed point but at the end of a 112 feet 50 Ohm coax line. The coax line usually impacts the complex impedance.  At 7.1 Mhz  the wavelength is L =  (300/7.1)*0.82 = 34.65 m (113.67 ft)   as  Belden RG8X  that I am using has  velocity factor of 82%.  This is within 1% the length of the coax so we can ignore the coax line as it is 2x half wavelength and    according the wikipedia in this special case.

 It looks like I need to analyze this discrepancy between the model and real antenna a bit more.   

Fig 9. AIMuhf Antenna Analyzer output

I looked at my coax cables to find out where this discrepancy is coming from.  I turned out to be more complicated than I thought.  I had 25 ft RG8X coax from ham shack to my feedthrough panel. Then I had 37.5 ft of  RG8U connected to 50 ft of RG213/U coax that was feeding the antenna.  Total length was 112.5 ft  and all this coax is supposed to be 50 Ohm but one  problem was that all these coax cables have different velocity factors. 

RG8X should be 82%  (I measured physical length 25ft  and using AIMuhf came to actual VF = 77%).

RG213/U  should be 66% and  RG8U should be 78% (foam).  I did not measure VF  of these two latter ones. 

I decided to simplify this coaxial setup and use only one type of coaxial cable.  I had 20 ft and 50 ft RG213/U cables that was just enough to get to the antenna feed point. After re-cabling this setup I measured coax length using AIMuhf TDR function, see Fig 10. below. I was surprised to see a big variation between 30 - 42 Ohms on the coax impedance that should nominally be 50 Ohms.  I re-calibrated AIMuhf  but got the same results.

Fig 10.  25ft + 50 ft of RG213/U coax cables

Using this newly installed 70 ft  RG213/U cable I measured the delta loop again. The SWR minimum has shifted 10 kHz lower to  7090 kHz.   

Fig 11. Delta loop SWR measurement.

Using  VK1OD.net RF transmission line calculator and taking Zmag 45.7 from above to Zin  I got the following results: 

RF Transmission Line Loss Calculator

Parameters
Transmission Line	RG-213/U
Code	RG-213/U
Data source	DSE
Frequency	7.100 MHz
Length	70.000 ft
Zin	45.70+j0.00 Ω
Yin	0.021882+j0.000000 S
Results
Zo	50.00-j0.39 Ω
Velocity Factor, VF -2	0.660, 2.296
Length	275.63 °, 0.766 λ, 21.336 m
Line Loss (matched)	0.355 dB
Line Loss	0.356 dB
Efficiency	92.13 %
Zload	55.12+j0.17 Ω
Yload	0.018142-j0.000056 S
VSWR(50)load	1.10

Once again  the  NEC2 model and reality seem to be far off  -- the Zload results above  55.12 Ohms +j0.17  does not seem to match with  140 Ohms predicted by NEC2 model.

Looks like this  investigation will continue.

In any case I did work several stations with this new delta loop antenna with good results.