The full wave loop may be the single best antenna a ham can deploy for multiple band operation.
Although this article is specific to my 80 meter full wave loop, the points apply to any horizontal full wave loop. I will make a note where any difference may exist.
Some of the advantages of a full wave horizontal loop vs other commonly used antennas:
- Closed circuit antenna designs like the fw loop are known to be more resistant to atmospheric noise and static than non-terminated designs
- You can get more wire in the air with a smaller overall footprint than a dipole for the same frequencies
- Does not require balance feed line for all band operation
- Can take advantage of balance feed system to provide resonant coverage for a huge portion of current and future amateur bands
- The fw loop is an excellent receive antenna system used by itself or combined with other antenna systems for diversity operation or short wave listening
- Does not need to be very high off the ground
Designing a full wave loop antenna system
There are a few things you will have to determine as you set out to design a full wave loop: first of all, an antenna needs to be designed as a system, it is the sum of all its parts.
So what parts are needed? The antenna itself, the feed-line, lightning protection, balanced to un-balanced transformer, and a wide range tuner rated for about twice the power you intend to operate at. You will also want a high quality peak-reading watt meter.
The Antenna
My antenna is designed for 80-10m operation. (Can also operate on 160m with reduced efficiency) It is 300 feet long and constructed from #14 coated and stranded copper wire. Since I use a balanced feed-line system I do not need to get hung up on trying to make it resonant on a particular frequency.
LB Cebik (W4RNL SK) wrote that one should put up as much wire as you have room for and run it. I would be inclined to agree. But that will only work with a balanced feed-line, if you intend to use coax, you will need to tune the antenna to resonance.
For a full wave loop this is done by using the following formula: 1005 / FREQ (1005 / 3.9 = 257.7′)
So how long to make the antenna then? Ideally, you would make your loop two or more wavelengths long for the target band. IE; 520 feet for an 80m loop. But following the Cebik rule, put up as much as you can and run it. I have room on my half acre lot for 300 feet so that is what I run.
A fw loop for it’s resonant frequency of operation will radiate broadside to the loop. For a horizontal loop this can be a problem. In the case of my 80m loop, it will be transmitting straight up to the clouds, a so called “cloud burner”.
Without getting into the weeds too deep here, there are a few points which cannot be ignored. Most hams cannot get a horizontal antenna for 80 meters high enough to radiate anywhere but straight up. An inverted-V can help a little but not much. The fact is almost any 80m horizontal antenna will be a cloud burner. This is why those chasing DX on the low bands will go to a vertical antenna system for a lower departure angle.
When a horizontal loop starts to get close to 2 wave lengths long the take off angle, or radiating pattern of the antenna begins to change. There are a few other tricks that can be done to affect the take off angle but they are beyond the scope of this article. Remember that adding length to enhance low band performance may impact the higher bands as well.
TL;DR: make the antenna as long as you can and raise it as high as you can in the air. Keep it as far away from trees as you can. This is not always easy, but do your best. Try and make the antenna as square as you can. (300 feet / 4 = 75′ per side) A triangle is OK too but a square is better. A circle would be best, but is logistically difficult at 80m and down. Most folks choose to corner feed the loop.
I’m leaving out a lot of technical information on loops here but if I don’t you’ll be here all day. I highly recommend the ARRL Antenna Book and ON4UN’s Low Band DX’ing for more information.
The Feed-Line
I recommend the balanced feed-line system over the coax.
I have tried two different types of feed-lines with my 80m loop: 450 ohm and 100 ohm.
When I first laid out this antenna, I decided to try a different type of feed-line initially: a 100 ohm line made by running two parallel runs of RG8X. This seemed like it would be a good fit since the 300′ loop has an impedance of about 100 ohms. Note there is no need to match the feed-line impedance to the antenna, but it seemed interesting so I gave it a go.
It worked out OK but the coax gets heavy and didn’t seem to offer any advantage over the standard balanced feed lines so I changed it while doing some maintenance on the antenna a few months later.
I am currently running 450 ohm feed-line on my loop and it’s working well as expected. I would likely try the DX Engineering 300 ohm line in the future as I have had good luck with it on other antennas and it is a little lower profile vs the 450 ohm and works the same in my experience.
Note this is a high power Cu clad steel feed-line not the cheap stuff used on your TV antennas.
Running balanced feed-line does require some consideration as it can be affected by metal objects and resonances. These are unique to the installation but are generally easy to work out.
I recommend running your balanced feed-line all the way into the shack as close to the tuner as possible.
Lightning Protection
I have found that installing lightning protection in the balanced feed-line works better than using protection in the coax. This also allows for the protection to be outside where it belongs.
I use and recommend this unit: https://www.dxengineering.com/parts/dxe-llsp
I use these on all of my balanced antennas and they are great, not only for the lightning protection, but also due to the bleeder resistors installed which help drain static and atmospheric noise to the single point ground system. You do have a single point ground system in your station right?
Balanced to Unbalanced Transformer
Also known as a “Balun”, this device changes the balanced feed-line from your antenna to an unbalanced connector typically a SO-239 for 50 ohm coax cable.
There are a multitude of different transformers that you could employ with a 300 foot loop and which one you choose will depend on what bands you will work the most and how much power you will run through the system.
Your basic choice will be between a 4:1 or a 1:1 transformation. In either case get one that will maximally choke common mode at your chosen frequencies for best results. A secondary choice is between a voltage balun, a current balun, or even a hybrid balun.
Many older tuners on the market feature a 4:1 Voltage balun for their balanced connectors. You should consider not using this and instead use an external Current Balun.
I have used and recommend the following among others:
The 1116d from Balun Designs is the one currently in use on my 300′ loop. Since it is a 1:1 with max choking on the lower bands, it’s a good fit for a low band loop on 160-20m. ( I have another antenna for the higher bands )
https://www.balundesigns.com/model-1116d-1-1-balun-optimized-for-low-bands-5kw/
I have used the Balun Designs 4116t on the loop as well and it seems a little better for 17-10 meters (being good on 80-20m as well) but I cannot use 160m with the 4:1 as it results in very low impedance that most tuners cannot cope with. The low impedance problem is also the reason the 1116d mentioned above isn’t as good with the higher bands as the 4116t.
For best performance on 80-10 meters, use the 4116t:
https://www.balundesigns.com/model-4116-4-1-hybrid-balun-1-5-54mhz-3kw/
You want the balun as close to the tuner as is feasible. Running long lengths of coax will introduce losses into your system so keeping the coax as short as possible will be ideal.
Mine use 4 foot jumpers made from LMR400 coax.
What Kind of Tuner?
The tuner built into your radio will not work. Most internal tuners max out at 3:1 and a wide band loop can see SWR well in excess of that.
Tuner choices are many and range from almost free to several thousand dollars. Starting at the lower end I would recommend a used manual tuner. Something like a Heathkit 2060(A) or a Dentron Super Tuner will be a great place to start. For an auto-tuner you could go with Palstar, Elecraft, or Flex Radio Tuner Genius among others.
I have run many tuners over the years. For a manual tuner I like a Dentron Super Tuner for up to 500w operation. For more power I used to run a Palstar AT5K-HP which could tune a wet noodle and do it at 3500 watts continuous power.
I have run two legal limit auto tuners: the Palstar HF-AUTO and the Flex Tuner Genius SO2R by 403A.
Both are excellent tuners, the Palstar is a better choice if you have several different radios as it is RF sensing and only requires a burst of RF to trigger a tuning cycle. Up to three antenna choices can be set by band only.
If you want full station automation, integration with the flex 8×2 antenna switch and built in SO2R operation the 403A Tuner Genius with the Antenna Genius cannot be beat.
This combo remembers tuning solutions for all eight antennas on all bands and switches them in as soon as you QSY. Very nice, combined with a solid state linear you can have instant power at the touch of a button.
Peak Reading Watt Meter
Any watt meter should work OK, but a few of them are really next level. (Cool and $$$) I highly recommend a peak reading watt meter. Many area AVG only, especially if they are built into your tuner
These two are my favorite:
Telepost LP-100A: The LP-100a is more than just a simple SWR/watt meter. It has additional modes no other watt meter has – vector impedance, dBm/RL, calibrated field strength and compression ratio.
It is essentially a real time antenna analyzer that monitors your transmitted signal and gives real time feedback.
Check out the website for more information: http://www.telepostinc.com/
Telepost LP-700: The LP-700 is a digital station monitor that has tons of great options to monitor your transmitted AF and RF. It is very useful to ensure linearity of your amplifiers as well.
Able to monitor four samplers automatically, it is very versatile with multiple amplifiers and antennas. In an SO2R setup you can track input power, output power, and amp linearity of both radios and amplifiers instantly.
Check out the website for more information: http://www.telepostinc.com/
Running the 80m Full Wave Loop
The 300 foot loop is the best overall antenna I have ever used. I can’t see ever being without a large loop in the future if I could help it. If you have the space, run a 520 foot loop with the 4:1 balun.
I can hear more stations on this antenna than I could on my 130 foot doublet. This shouldn’t surprise anyone since it’s more than double the wire in the air. But it’s also a common experience reported by those who are “loopy” that these antennas HEAR very well.
What about transmit performance? I can say that I always get many positive comments on my signal.
To me, that is what matters. Other stations reporting good performance and breaking pileups.
Whether 15 watts, or 1500 watts, the 300 foot loop is a beast.
What about 160 meters? The 300′ loop will play on 160m, but you have to understand it is not the most efficient antenna for top band. If you wanna get on and chat with the local’s, it will work fine but will likely require you to use a 1:1 balun which may create issues for 17-10m operation but could probably be worked around.
60 meters? Yes, the 300 foot loop works very well on 60 meters.
I have worked all over North and South America on 10 meters. The loop has a lot of gain on 10 but will be a bit peaky due to the number of lobes on that band.
As I mentioned above, I run 44 foot doublet for 40-10m operation as a primary and use the loop for diversity reception and a second antenna since it sometimes hears better than the doublet.
My tuner can auto-tune all amateur bands to 1.25:1 or better from 160-10 meters. It’s hard to ask for more than that from a single antenna, especially when it is pretty inexpensive and easy to build.
Get one.