The One About the Loop Antenna
In a message dated 3/31/2014 9:24:10 A.M. Central Daylight Time, email@example.com writes:
With your experience with loops, I would love to hear what you think about his approach.
I read the attached article by Steve, WA5FRF. Although I'm not a big fan of computer modeling programs, they certainly give us pretty good insight into the expected patterns of radiation from antennas we are thinking about building. Steve's loop radiation pattern predictions are close to others published in QST and elsewhere. Doug DeMaw put his in QST in 1990 and later that year the plots were revised in "Technical Correspondence." W9SR published his some years after that.
All the plots are similar and these authors plus many others make assumptions about the loop's performance based on the plots. But, as they say, the proof of the pudding is in the eating.
I was enamored by 160 from the time I got my license in the mid 50's. By the early 60s I had the room and wherewithal to put together a pretty good 160 station in Minooka Illinois. From then until the late 80's I used 130' verticals for transmitting and had various beverages, loops and long dipoles to receive. Although I've always been a casual DXer, I worked a lot of exotic DX on 160 in those 15 years or so.
My first use of a horizontal loop was a 12' high full wave for 20 meters. I put it up on a cold, damp fall day in Channahon Illinois about 1985, thinking it might help Joyce, WB9NUL, my XYL, hear close-in mobiles on the county hunting net at 14336. She regularly ran as net control for 8 hours at a time. All her other antenna choices at that time were pretty high, ergo, low angle. Conventional wisdom said the loop was a cloud warmer, ergo, good for close-in if the ionosphere cooperated.
It performed as advertised. In fact, it heard everything except noise. I was really impressed. In fact, she would occasionally transmit on it to be better heard close in. If she would forget to go back to her 70' high beam or the small conical monopole on top of the 130' vertical, everything was fine for stateside. Then when one of the VK's or EU stations called in she had to switch back. OK, that figures.
In 1989 I took early retirement and we moved to the Harlingen area. We had decided to simplify our ham station along with the rest of our world. We got rid of all tube equipment and went solid state, the biggest amp being a Yaesu FL7000. We said that all antennas would be confined to our one-acre lot. We brought down only one of our towers, a sixty-foot Spaulding (Rohn BX these days), and one beam, a Mosley TA-33. I figured we'd put the beam on the tower for Joyce and I would Gamma match it as a top-loaded vertical for 160 & maybe 80.
That left the need for a receiving antenna for DX on 160. As I thought about pennants and flags the experience in Channahon came back to me. Bottom line... In the spring of 1990, I put in four telephone poles and installed a 520' square horizontal loop at 50' high. It would be my receiving antenna for 160 and a utility antenna for the upper bands. It was fed with a homemade open wire feedline and a balanced output tuner.
When it was about finished the article by Doug, W1FB came out in QST. We were long-time friends, so I called him at his retirement home in Luther Michigan, and told him about my project and what my objectives were. He said, "Well don't ignore its potential as a transmitting antenna for 160."
I told him that I was looking at its reputation as a cloud warmer. He asked if I remembered an old-timer W8 that died some years before... and I did. He said that the old boy always used a full wave loop that surrounded his urban home. I commented that now I knew why he was so strong in W9. Doug said that back in the day that guy would regularly work VK's & ZL's and such on 160 when no one else could get through, let alone hear them. I took that with a grain of salt.
The loop was quiet. It seemed to hear very well, but by now it was summer, and 160 was getting bad with lightning static. The tower would go up in the cooler months so meanwhile, Joyce began using the loop on her nets on 40, 20 & 15. By the end of summer, she told me to sell the TA-33. She said that she had never heard and been heard as well as she was now. OK, I'll do something else with the tower vertical.... but later as I was very busy. We had a consulting business with clients all over North America at that time.
When the first contest ran that fall I told Joyce I would check out the loop's performance as a receiving antenna on 160. That evening I sat down with the cans on and listened as things got going. I was pleased to hear a couple of G's calling CQ test. I next heard an EA6 calling. Joyce asked what was I hearing and I told her. "This thing will do the job when I get the transmitting vertical up." She asked, "Who are they working, East Coast, as usual?"
I said, "No, mostly each other." She asked why I wasn't calling them. Well, my answer was based on an ego problem. I had, for many years, been one of the big guys in the band. Now here I was with 500 watts and a cloud-warmer antenna. I didn't want to embarrass myself. That was really stupid of me, but I guess my pride ran away with me. Joyce said, "If you're not going to call 'em, move over!"
So I called the EA6. He came right back. It had to be a fluke... strange conditions... something odd. The thing is, I had a lot more of those flukes that night.
I got up early the next morning to see if I could hear a JA. I was listening in that little segment around 1910 to which they were confined at that time. I heard JA's working with each other. The first time I heard one of them send "QSX 1808" I answered. He came right back. I worked 23 JA's that morning with my cloud warmer. I didn't get any "599 Booming signal" reports but I was very competitive.
That kind of thing was the norm from then on. The vertical went up a couple of months after that contest weekend. In all the time at that location no other receiving concoction ever outdid the loop.
But now let me summarize my 25 years using a big loop and an efficient vertical. I can identify a pattern of dominance between the two antennas. For about 5 years, the loop was the best of the two about 75% of the time for DX, both receive and transmit. Then for about a year, it was even. Then for about 5 years, the vertical dominated 75% of the time on transmit, but I had to listen on the loop.
This pattern has continued to the present where we are on the declining side of a vertical 5 years. This is most evident on 160, somewhat evident on 80, and unnoticeable above that. The loop has always performed surprisingly well on 40 through 10... and even 6 meters. Of course, anything works well on 6 when the band is open. We can hold our on in pileups and usually hear things better than most on 40-10.
I've spoken with a number of friends around the world that study and measure signal arrival angles, propagation, and the Earth's magnetic fields. Most of these guys say that arrival angles vary greatly on all the bands all the time and have some scientific reasons I don't understand to explain the 5-6 year patterns of angle dominance. Steve, VK6VZ sees the same pattern as I see.
All this leads me to say that the general assumption that only very low-angle radiators are good for DX is not substantiated in practice. I guess I had about 150 countries on 160 when we moved to the valley... from about 35 years of trying. I have 254 now and it's many times tougher to add new ones as you go along. The loops have been a big part of the increase in numbers.
Now to your other questions. When we moved about 8 miles to rural Santa Rosa, we had a chunk of farmland behind the house. I had four 70' poles installed and put up the 1150' loop. All I can say about it is that in the nearly 7 years it was up I never sensed an improvement over the 520' loop. We now have 565' at 61' and it works the same. This one is supported by aluminum. The big one just had more lobes and more nulls, but that makes no difference by the time the energy is bouncing around in ionized layers. When all gets said and done, we can look at the models, patterns, and propagation numbers and it will still be a crapshoot whether the contact will be made.
As for the tuner issues... I was a stickler about baluns for many years. They had to be 50-ohm one-to-one baluns on the input to a balanced tuner. Those tests you mentioned made me realize that my mindset was developed around working with powdered iron cores or toroids and even bifilar or trifilar air-cored baluns. As it turns out, modern ferrite mixtures are being used that allows a much wider tolerance of both frequency and input impedance. Read the Sevick book "RF Transformers." Those tests proved to me that a well-designed single-ended (unbalanced) tuner with a ferrite-cored balun on the output can be just as efficient as even a resonant link-coupled unit.
The difference in tuners that is most troublesome today is the lack of sufficient capacitance on either side of a "T" or an "L" circuit to produce an efficient match. Everyone prays at the SWR altar. If it says 1:1 they figure it's as good as you can get. That's a big failing of many commercial tuners, including Palstar.
If you haven't played with the simulator I mentioned in the text, go to that site, read what he has to say and put in some high and low impedance numbers on 160 for instance with various reactance number and then push the auto-tune button. Take a look at the efficiency and the loss figures. Then raise the capacitor values and do it again. You'll get the idea. I’ve seen tuners easily achieve 1:1 SWR and be only 30% efficient. Lookout! Something is going to catch fire or melt.
Yes, the double balanced "L" tuners were home brewed. I have built 40 of those over the years and still have some of them around here. I have both high pass & low pass versions. They are good tuners. Nevertheless, the MFJ998 and a well made balun are just as efficient and oh so quick and easy. The 998 isn't as pretty as the Palstar but it's infinitely faster and has adequate capacitance for an efficient match. As I get older there are fewer and fewer things I'm willing to stand in line for.
Oh, and one other point, Sandy.... Steve's emphasis on resonant frequencies of the big loops is unwarranted. The resonance of an all band, balanced fed antenna is inconsequential. The sum total of the varying feed line connection impedance to the loop across the frequencies of operation, the characteristic impedance of the transmission line, the length of line and other factors like bends & proximity all contribute to what the tuner has to deal with at the shack end of the feed line.
I had a hard time getting Brian to unhitch from the concept he had about feeding a balanced multi-band antenna. Mainly, he thinks a loop has a specific feedpoint impedance. That's only true if you understand that it will be a different specific impedance at each and every frequency at which it is to be used. See the plot of the 1150' loop in that tuner write-up. The variations are extreme.
Secondly, he assumed the characteristic impedance of the feed line has to be the same as the loop feedpoint impedance. Also not true... and damned near impossible anyway, HI. The balanced feed line is nearly lossless so standing waves don't cost us anything on the line.
When the load at the shack end of the line is matched, currents and standing waves are equal and of opposite phase on the line, therefore they cancel each other and no radiation (or reception) will occur from the line.... no loss. Meanwhile RF current will be delivered to the antenna so a field will be produced that "loses" energy on each phase reversal of that alternating current. Those "losses" are radiated signals. The feedline is designed for no loss of energy and the antenna is designed for maximum loss of energy, if you will.
As long as the tuner has adequate component values to provide an efficient, balanced match, then maximum power will be delivered to the antenna for maximum radiation field... whether it's resonant or not. It's a different story for an unbalanced fed standing wave antenna like a vertical monopole. In that case, the reactance needs to be tuned out at the feedpoint, in other words, resonance needs to be achieved.
Guess that covers it, Sandy. Let me know if it doesn't. I'll try to stop by 1900 now and then when I finish with my DX hunting cronies on 3796 in the mornings.
1. Mobile Antennas
2. Low Ban Antenna Survey
3. Actual Measured Performance of Short, Loaded Antennas - Part 1 (PDF from original article published)
4. Actual Measured Performance of Short, Loaded Antennas - Part 2 (PDF from original article published)
5. Creeps and Weirdos (Tongue in Cheek)
6. The All Band Antenna
7. The Ultimate Ultimate Transmatch
8. The Invisible 20-Meter Beam
9. The Hidden Antenna
10. The Loop Antenna