Shielded loop antennas for 160 meters/AM broadcast band

[wx3k]

Does anyone make a shielded loop antenna for the AM broadcast band that is broadband enough to cover 160 meters ? Can anyone recommend a source ?

[Steve - WB3HUZ]

It appears you are looking to buy. Check Wellbrook, Kiwa, and Palomar for starters. I know Palomar makes a ferrite based loop systems has different heads that plug into a base/preamp box. The heads cover different frequency ranges up to 6 MHz.

A shielded loop is pretty easy to build. All you need is some coax and some supports/spreaders. I build one with RG-6 and a few pieces of small PVC pipe and a four-way connector to make the support/speaders. A small cap tunes the thing. See the attached drawing. I built only the larger one shown. It worked pretty well. There are some designs out there with more complicated feed and impedance matching systems that will give better results.

[Pete, WA2CWA]

Like Steve, I made one similar to the larger one shown in his picture. Bought a 10 foot piece of PVC pipe, cut it in half, took a square piece of wood for the center, and attached the pipes with some U-boots. I used some junk RG-59U that I had laying around for the loop. Mounted the variable in a small plastic box at the bottom end of the loop. Stuck a small section of PVC pipe in the bottom end and mounted on a rotor. Have some landscape railroad-type ties in the back yard and mounted the bottom of the rotor to them. Used it for several years on 160 and it even covered the upper end of the broadcast band. Unfortunately, in a fit of lawn mower misdirection last year, I chopped up the rotor and coax that was headed back to the shack.

[k4kyv]

I built one like the larger one shown, years ago, in the 1980's, when my shack was located in the upstairs room in the house.  I didn't even make it rotatable.  I just hung it on the wall like a map.  It was considerably quieter than the quarter wave vertical on 160.  I can run the capacitor out to minimum and make it work on 75.  Probably would need more capacitance to cover the BC band.

I don't find the null useful for skywave signals, but it can be effective for nulling out locally generated noise.  Even then, the null is extremely sharp, which is why mine worked so well as a general receiving antenna in fixed position in the house.  I relocated it to the present shack, and now have it on a rotatable base.  I constructed mine using "spider" construction, somewhat like one element of a cubical quad.

It always amazed me that I could hear better with that little hunk of coax than I could with the 127' series fed tower.

But when I put up the beverage, it is head and shoulders above the loop in the direction of the main lobe, unless the power lines 1/2  mile away in that direction are acting up.

[wx3k]

I appreciate the responses.  I am just a bit busy to take the time to wind up a loop myself. I was hoping to find something already built that I could tweak but I guess it isnt that difficult to find some parts kicking around the house and build it.

Question: I read that using double shielded or foiled coax works significantly better for nulling out noise. Anyone note this in their own experiences ? I own a MFJ 1026 signal enhancer and want to use the loop as a sense antenna to null out some nasty sign ballast noise on 160 meters.

[Steve - WB3HUZ]

Double-shielded makes no difference. The shielding is not used or the antenna would receive nothing. The shielding on the coax does potentially improve the balance of the antenna which will in turn produce the classic figure-8 patter of a small loop. The nulls in the loop will be much sharper than if the loop was less balanced.

However, if all you want is a sense antenna to pick up the ballast noise, a simple monopole (a.k.a. any old piece of wire) as close to the noise source as possible will work. A loop is not needed and would probably be less effective.

[W1VD]

If the loop is built and fed in a balanced manner and kept away from things that will unbalance it (loop mounted outside in the clear for example) shielding is unnecessary.

The 10' loop below is 14 turns unshielded wire and is series tuned. Coupling is via a ferrite toroid transformer. The turns ratio of the transformer in conjunction with the 50 ohm preamplifier controls the antenna Q. It's designed for LF work but the principles apply at other frequencies.     

[K6JEK]

I appreciate the responses.  I am just a bit busy to take the time to wind up a loop myself. I was hoping to find something already built that I could tweak but I guess it isnt that difficult to find some parts kicking around the house and build it.

Question: I read that using double shielded or foiled coax works significantly better for nulling out noise. Anyone note this in their own experiences ? I own a MFJ 1026 signal enhancer and want to use the loop as a sense antenna to null out some nasty sign ballast noise on 160 meters.

You should use the loop as the receive antenna and use the transmit antenna as the noise source.  The loop has deep nulls broadside. If you can and you probably can, aim that at the LHNG (Local Horrendous Noise Generator). Phase in enough of the the other antenna to quiet it down a bit more.  I did that for years with an MFJ 1025 until I realized I was getting almost all of the benefit from the loop alone.  I now use two loops each with its null aimed at the LHNG and the two phased to peak and null signals.  I've upgraded to a DX Engineering NCC-1 phasing unit too.

Here's a recording of the two loops in action.  The loops are a Wellbrook 1530 and a straight-from-the-ARRL Antenna Book Doug DeMaw tuned looped. The tuned loop works just as well as the Wellbrook and it cost nothing. But it is tuned.  On the recording you'll hear Cliff, N6ZU, first received on the dipole then the loops peaked, then nulled (while I fooled around with the nulling) then peaked again, then back to the dipole.  I think you'll hear why I like the loops and the NCC-1

[Steve - WB3HUZ]

Very nice! Killed that noise.