W8WWV - The 3805er Receiving Loop

Greg Ordy

Ragchewing on 75 meters with the locals (several hundred miles) can be a lot of fun. The summer months, however, with their high static and noise levels, can be brutal on the ears. I have found that a small receive-only loop antenna can be used to make the situation much more tolerable. This page describes an easy-to-build loop that I have built several times for myself and friends. We tend to congregate on 3.805 MHz, so I call this version the 3805er.

The perimeter length of the loop is 12 feet, and it is in a diamond shape. It is approximately 4 feet wide and 5 feet tall. It is certainly possible to use the loop in the shack, although I find the performance to be better when the loop is outside.

Loop against garage doors

A 3805er up against the garage


In order to improve reception, it is necessary to improve the signal to noise ratio. Common 80 meter antennas such as low dipoles, inverted Vees, or verticals, suffer on receive because they have a nearly omnidirectional response pattern. While the desired signal is arriving from one direction, noise can arrive from all directions. A loop antenna such as the 3805er improves this situation because it has broadside nulls at very low wave angles. Most radio signals within several hundred miles tend to arrive at much higher wave angles (above 45 degrees), and at those angles, the loop response is nearly omnidirectional. Often times, noise is relatively local, and arrives at a low angle. The loop can be oriented to reduce the reception of this noise. Less noise, same signal, improved signal to noise ratio.

Technically, the 3805er is a single-turn shielded loop antenna. This type of loop is described in the ARRL Antenna Book, and the ON4UN book. Some sources state that the shielded nature of the loop provides additional noise immunity by shorting the (noisy) electric field to ground while responding to the magnetic field. Because of that belief, this type of antenna is also called a magnetic loop. I'm not sure if I believe that particular theory, but in any case, the loop can improve reception of short-distance radio signals.

In my experience, this type of antenna is most useful for short-distance work in the summer months. I do not consider it to be a good DX antenna, although it has been used for DX by some. When winter comes, and 80 meter noise tends to drop naturally, the advantage of the loop often disappears. Please note that the 3805er is a receive-only antenna. In order to use it, it is almost a necessity that your radio provides a separate receive antenna input jack. This antenna, like most receiving antennas, will have very low output compared to transmit antennas. In some cases, a preamp can be useful. It is entirely normal that when using this antenna you will have a nearly S0 noise level, and the station you are working will barely move the meter. What will be missing is the deafening noise crashes and static bursts that contribute to ear fatigue.


The antenna design is certainly not original. The loop is constructed from small diameter 50 ohm coax, such as RG8X. Any coax diameter or characteristic impedance can be used, so long as an appropriate capacitor resonates the antenna at the desired frequency.

In order to preserve the broadside nulls, the literature suggests that the length of the loop wire should be less than 0.1 wavelength. That limits our perimeter length to no more than 25 feet. I chose the 12 foot length since it naturally falls out from using standard 4 foot dowels for spreader arms. I also wanted a loop that could be used inside as well as outside, and the 4 foot dowel-based form is light and not difficult to move around.

I use a small trimmer capacitor to resonate the loop at the desired frequency, 3.805 MHz. A loop built following my dimensions and geometry has an inductance of approximately 5.3 uH at 3.805 MHz. This is around 125 ohms of inductive reactance. A 325 pF capacitor will be needed to achieve resonance. I happened to have a bag of 270 pF hamfest trimmers, so I parallel an 80 pF silver mica capacitor across the trimmer to get up to the needed value. I also usually include a small step-up transformer to match the low loop impedance to the 50 ohm feed line. This is an optional part of the design. I do find that the addition of the transformer increases loop output. The transformer is wound on a 1/2 inch ferrite core. Any mix suitable for the frequency will do (#75 (best), #43, etc.).

loopant.gif (5313 bytes)

3805er Schematic

Parts List

In order to build your own 3805er, collect the following parts:

Parts List

Part Description


12 feet of 50 ohm mini coax (RG8X)


PL-259 coax connector with mini coax insert


4 foot dowel rod, 5/8 inches in diameter


3/4 inch aluminum tubing, 3 inches long


3/4 inch aluminum tubing, 18 inches long


2 3/4 inch X 2 1/8 inch X 1 3/4 inch aluminum project box


1/4 inch grommet


1/4 inch X 20 stainless steel bolt, 2 inches long, and locking nut


1/4 inch screw eye


#8 X 1/2 inch long stainless steel machine screw


terminal strip, 5 lug


variable trimmer capacitor, approx. 350 pF (see text)


cable ties, small


cable ties, large


1/2 inch ferrite core toroid, #75 or #43 material


solid hookup wire, #22

2 feet

electrical tape

8 inches


Loop parts

3805er Parts

In addition to these parts, and common hand tools, you will need a drill and several different drill bits. One bit makes the hole for the grommets. For a 1/4 inch hole grommet, a typical drill bit size is 3/8 inch.  You will need a 1/4 inch drill bit for the 3 inch aluminum tubes and the 1/4 inch X 20 stainless steel bolt that joins the tubes and dowels. Finally, you will need a drill for the #8 screws that attach the project box to the 18 inch aluminum tube. The bit needs to be slightly smaller than the screw so that you can use the screw in a self-tapping manner. The diameter is approximately 1/8 inch, but size it according to your screws.

If you wish to accurately resonate the antenna, and optimize the output of the antenna, you will need an antenna analyzer that measures SWR as a function of frequency.


Here's how I assemble a 3805er.

Dressed coax ends and antenna center

Dressed coax ends and braid removed from center of antenna coax

Loop center

Center of the loop, showing 3 inch tubes around dowels with bolt as center pin

Checkout and Adjustment

Connect the antenna to an antenna analyzer. Set the analyzer to the desired frequency on the 80 meter band. If your analyzer provides antenna reactance (X) data, adjust the trimmer for resonance, that is, reactance equals zero. If not, adjust the trimmer for the lowest SWR. If you are using the matching transformer, you should be able to achieve a 1:1 SWR. I add or subtract turns on the secondary until I get a 1:1 SWR (once I am at resonance). If you are not using the matching transformer, then the point of lowest SWR is probably not the point of resonance, but they should be close.

Assuming the addition of a transformer to match the loop to the coax and provide a 1:1 SWR at resonance, the 2:1 SWR bandwidth was measured to be 36 KHz.   Once you get approximately 50 KHz away from resonance, the loop signal (and noise) output will drop by several dB. This is not really a problem, as most all receivers have more than enough gain to compensate. Still, if you want maximum output from the loop, you should adjust the resonance point to your desired frequency.


It is almost a necessity to use a radio that has the ability to accept a separate receive antenna. Fortunately, this feature is becoming standard on most all recent vintage radios.

There are a number of ways to mount the antenna. For inside use, you could simply lean it up against a wall. The antenna should be kept vertical, and potentially rotated to null out local interference. I had a used wooden spool for holding coax that provided a good mount though the center hole. I simply laid the spool on its side, and placed the 3/4 inch aluminum tube base in the spool hole. You could also take a piece of wood, such as a foot long section of a 2X6, and drill a 3/4 inch hole in it to act as a base.

Outside, the simplest mount would be to push the 3/4 inch aluminum tube into the ground. the only down side of this approach is that the tube will get filled with dirt. A variation on the this theme is to drive a short length of 5/8 inch tubing into the ground, then slip the antenna tube over the 5/8 inch tube, since the two sizes telescope. Now the antenna tube will stay clean, and the antenna can be easily rotated. I have also used an elastic cord to strap the antenna to a deck railing. The antenna could also be hung from a low tree branch with a short length of strong string.

Some sources suggest putting a loop on a rotator so that the nulls can be easily moved. I have never had this setup, so I cannot comment on its value. It is interesting to use the loop in the shack, however, where it can be turned by hand. I find this especially interesting right before sunset. So long as there is daylight, it's a good bet that most signals are arriving via ground wave, at a very low angle. As you rotate the loop, the broadside nulls will be quite obvious. There should be very little advantage in raising a loop high off of the ground. Operation at ground level is just fine.

I prefer the loop outside. I have mounted mine on a wooden deck handrail, a few feet off of the ground. This gives me the best results. Inside my second story bedroom shack, there is too much local noise (computers, TVs, etc), and I believe that I lose a few dB of signal (and noise). This is compared to being out in a flat field, almost 100 feet from the nearest building.

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Last update: Monday, September 30, 2002 02:41:07 PM
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