Wellbrook Loop: About The Mast

[ridgescan]

My Wellbrook is, as some here know, way up on the roof of the building I live in. It's 50' up. It performs well up there although that kind of height is not ideal for what this antenna's designed for, as it does have more noise up there than I like in HF.
My question is this though: it's on a tripod. It has two masts; one from the tripod to the rotator, and one from the rotator to the loop. For windage, I got a nice plastic-coated pine wood dowel 2 years ago for the tripod-to-rotator run and it's doing very well. But the mast between the loop and the rotator is metal.
If I get that metal mast out and put in another coated wood dowel, ya think this might even slightly improve S/N? I read that the loop performs better if it's sort of made to "float" in the air.

 

[ka3jjz]

There is a potential here - and I'm not saying this is actually happening, it would need to be tested - that the metal pipe going from the loop to the rotator may be unbalancing the loop somewhat. I would think It would be good practice to keep metal as far away from the loop as you can.

The only way to know for sure is to try it. We can all argue theory all day long, but it's the actual practice (results) when the rubber hits the road..

I have also been reading somewhere - I can't put my finger on it at the moment - where folks have put line isolation on the coax to remove the chance of the coax conducting common mode noise. As far as I know, the jury is still out on this one.

Mike

 

[ridgescan]

There is a potential here - and I'm not saying this is actually happening, it would need to be tested - that the metal pipe going from the loop to the rotator may be unbalancing the loop somewhat. I would think It would be good practice to keep metal as far away from the loop as you can.

The only way to know for sure is to try it. We can all argue theory all day long, but it's the actual practice (results) when the rubber hits the road..

I have also been reading somewhere - I can't put my finger on it at the moment - where folks have put line isolation on the coax to remove the chance of the coax conducting common mode noise. As far as I know, the jury is still out on this one.

Mike

Thanks Mike with that I will go and get that dowel and when I finally deploy it, I will post any change. Also I have been thinking about the coax thing, as well as the rotator cable.

 

[ka3jjz]

I found such a reference on the Active Loop Antenna group on Facebook. Here is the unit that is being discussed...

MFJ Enterprises Inc.

Of course for receiving apps the 1.5 kw power rating is completely unnecessary, but it gives you some idea of what was being used in the test...Mike

 

[ridgescan]

Can't beat that price either. I did an "Ugly Balun" at the loop end a few years ago; 21 turns of the feed line around 6" PVC but I doubt it's half as effective as that MFJ with the ferrite core.

 

[iMONITOR]

On a related note, I see numerous loop antennas mounted directly on top of a rotor (not to mention the metal railing that this rotor is mounted on in the picture below). I've wondered if this would also affect the properties of the antenna with all that metal in such close proximity?

 

[prcguy]

I don't think small amounts of metal like a mast under a loop matters much as its only a tiny fraction of a wavelength in size. I think a good common mode choke near the loop would be about the best you can do keeping noise and gunge picked up on the feedline from getting to the antenna. I've gone from mediocre choke baluns to very effective ones on other types of antennas and have seen noticeable reduction in my noise floor.

An ugly balun is not very good for anything except for storing excess coax. Here is the best receive only common mode choke I know of and it targets the 500KHz to 10MHz range: https://myantennas.com/wp/product/cmc-0510-r/

 

[vagrant]

Here is the best receive only common mode choke I know of and it targets the 500KHz to 10MHz range: https://myantennas.com/wp/product/cmc-0510-r/

I am curious if that is what you are using with your Hexbeam or just your OCF?

 

[prcguy]

No, the choke in my link is for receive only and I use one at the moment on a 3ft VLF/HF home made loop. I have a home made choke on my Hexbeam and I forget what ferrite mix I used but it is tailored for 20 through 6m. My 80m offset center fed has a home made version of this four core monster: https://myantennas.com/wp/product/cmc-230-5k/

At the radio end I use either the MyAntennas CMC-130-3K or an older version CMC-330-3K on most of my HF feedlines. There is literally a truckload of ferrite around here.

I am curious if that is what you are using with your Hexbeam or just your OCF?

 

[vagrant]

Thank you for that. I fiddled with ferrite beads and what not in the past. As my knowledge has increased I recently had a box full delivered for some serious tasks.

 

[ridgescan]

I don't think small amounts of metal like a mast under a loop matters much as its only a tiny fraction of a wavelength in size. I think a good common mode choke near the loop would be about the best you can do keeping noise and gunge picked up on the feedline from getting to the antenna. I've gone from mediocre choke baluns to very effective ones on other types of antennas and have seen noticeable reduction in my noise floor.

An ugly balun is not very good for anything except for storing excess coax. Here is the best receive only common mode choke I know of and it targets the 500KHz to 10MHz range: https://myantennas.com/wp/product/cmc-0510-r/

Thanks prcguy-you pointed me to that a while back but I never acted upon it. I need to get that one for both antennas.

 

[nanZor]

You can always test it with a battery powered drill motor (no drillbit need be inserted.)

Have a friend pull the trigger on the drill motor some distance away from the loop. Rotate your loop, crank up the gain etc so you can hear the broadband hash. It may be much stronger down near the broadcast band, so choose a frequency that you can hear it, but it isn't totally overloading the receiver.

Rotate the loop to see if you can null it out, or heavily attenuate the noise with the loop perpendicular to where your helper is standing. If the helper moves, be sure they notify you.

In ideal conditions, directly perpendicular to the loop is the correct response. If you can't seem to null it out no matter where you rotate the loop, you may be suffering from common-mode ingress. Here is where an inline choke near the feedpoint can help, and restore the pattern

 

[ridgescan]

You can always test it with a battery powered drill motor (no drillbit need be inserted.)

Have a friend pull the trigger on the drill motor some distance away from the loop. Rotate your loop, crank up the gain etc so you can hear the broadband hash. It may be much stronger down near the broadcast band, so choose a frequency that you can hear it, but it isn't totally overloading the receiver.

Rotate the loop to see if you can null it out, or heavily attenuate the noise with the loop perpendicular to where your helper is standing. If the helper moves, be sure they notify you.

In ideal conditions, directly perpendicular to the loop is the correct response. If you can't seem to null it out no matter where you rotate the loop, you may be suffering from common-mode ingress. Here is where an inline choke near the feedpoint can help, and restore the pattern

"no drillbit need be inserted"<<<LMAO! You a funny guy hertz
interesting test though-thanks

 

[nanZor]

I use the same technique with other antennas too to check for common-mode problems on the transmission line.

With me in the shack, and a drill-motor helper outside, (important - battery operated only) I have them crank up the drill, but have them run it very close to the coax moving along its length.

Of course I'll hear the noise from the antenna itself, but what I'm looking for are hot-spots where the noise from the motor just pegs the s-meter, or is substantially stronger than from the direct reception from the antenna. Sometimes I'll crank the rf-gain down, and do the test again to really narrow down any hot-spots along the coax.

 

[prcguy]

What's the pay grade for that kind of helper? I may not be qualified but I can stare at coax and antennas all day long without complaining.

I use the same technique with other antennas too to check for common-mode problems on the transmission line.

With me in the shack, and a drill-motor helper outside, (important - battery operated only) I have them crank up the drill, but have them run it very close to the coax moving along its length.

Of course I'll hear the noise from the antenna itself, but what I'm looking for are hot-spots where the noise from the motor just pegs the s-meter, or is substantially stronger than from the direct reception from the antenna. Sometimes I'll crank the rf-gain down, and do the test again to really narrow down any hot-spots along the coax.

 

[prcguy]

If anyone is curious what a good common mode choke can do for your noise floor, this video is similar to what I experienced. If you play the point where he inserts the choke you can see the noise went from about S8 down to about S6. You won't get that with an ugly balun or some ferrite beads slid over coax.

https://www.youtube.com/watch?v=CUS_HeDMdOU

No, the choke in my link is for receive only and I use one at the moment on a 3ft VLF/HF home made loop. I have a home made choke on my Hexbeam and I forget what ferrite mix I used but it is tailored for 20 through 6m. My 80m offset center fed has a home made version of this four core monster: https://myantennas.com/wp/product/cmc-230-5k/

At the radio end I use either the MyAntennas CMC-130-3K or an older version CMC-330-3K on most of my HF feedlines. There is literally a truckload of ferrite around here.

 

[majoco]

prcguy said....

I may not be qualified but I can stare at coax and antennas all day long without complaining.

Way back in the 60's I was a tech with the UK CAA installing instrument landing systems, whenever we had a problem with the antennas we would call for "Jim", who would duly turn up, listen to our problem, say "Hmmmm". Then he would fill his pipe, go outside and stand in front of the antenna, puff on his pipe for a while then come back inside and say "have you checked so-and-so" - more often than not he had the solution. Usually cost us a lot of beer too!
.

 

[ridgescan]

I just put up the wood mast today. I am happy to report a significant improvement in HF! Much more signals coming in that were virtually invisible under the noise floor. I counted 23 Asian stations alone that I can at least see on the waterfall or copy. Starting this evening, nights are gonna be fun with the improvement.
In MW, I see signals are 3-4 s units down from where they were. Still get 'em good though.
I have a feeling the loop was coupling to the rotator's electrical cable via the metal mast. I had a gut feeling about that for a while.
Now I have the loop working as-designed
Also by the way, it's a lot quieter from 2-5mHz!

 

[ridgescan]

To add, much better nulling. Better directionality than before. Much better selectivity in MW than before. The 50' height I thought was affecting the loop's performance, was not. Even at 50', it is doing what it's supposed to do. Been missing out all this time.
I may have had a unique-to-my QTH with the metal mast and the 50' of rotator wire, but I think Andy at Wellbrook should advise his customers to not use a metal mast especially if they are using a long length of rotator wire.

 

[mancow]

Should the CMC go at the antenna or in the shack near the radio?