VHF/UHF ant as rx-only HF using coax braid?

[nanZor]

I know this has been asked before, but couldn't find the thread.

And I know the answer - the vhf/uhf elements don't mean much on HF, and what you are picking up on HF is mostly coming from the common-mode of the coax transmission line braid.

But has anyone tried to *control* it with HF inline chokes, ala Myantennas, Palomar, Balun Design's llc and so forth?

I don't have a discone up, but here are my thoughts about a possible test. Lets say you have a discone, or other uhf/vhf antenna up at 30 feet or so. And you've listened to it on an HF receiver, where it is basically a noisy random wire from the common-mode of the braid snaking all over the place and in the shack.

So....

1) Disconnect the discone / vhf/uhf feepoint and attach a 16 foot coax jumper.
2) At the end of this jumper, put in an inline tubular ferrite choke. Perhaps make it a nice one from Myantennas, Palomar, Balun Design's llc etc. We are trying to force the choked off braid to be a vertical wire - but controlled in length so you don't have a funky pattern.
3) If necessary place another HF inline ferrite choke near the hf receiver.

My thought here is that since the chokes typically become ineffective at 30 to 54 mhz or so, they may be invisible to your existing vhf/uhf setup.

However, when used on your hf receiver ports, you now have your vertical coax braid section doing all the work with the common mode, and perhaps the only thing the vhf/uhf elements are doing now is just a little loading.

I've never tried this myself. And of course we are talking rx-only, and not even concerned with overall matching efficiency - just enough to get band-noise above your receiver's own noise floor when you insert and remove the coax.

Never tried it - thought I'd throw it out there for those who only have a scanner antenna, and don't / can't put up another antenna for hf.....

 

[prcguy]

I occasionally use a Discone or a clip lead for quick HF reception and it can work just fine. A VHF/UHF size Discone would equate to an E field probe at HF with an impedance of many thousands of ohms or more hitting a low impedance coax and loosing a good amount of signal in the process. I don't believe the coax is picking up very much compared to the hot disc element and if I get some time or if this rain stops I'll stick a good choke on a Discone and try an A/B test.

 

[nanZor]

It would be an interesting test, but surely don't put the HF choke right at the feedpoint if you want to incorporate the controlled common-mode vertical.

That's why I was thinking of temporarily hanging the choke at the end of a 12-16 foot jumper or so, and reconnecting between the feedpoint and the rest of the feedline and see what's up.

Part of the main emphasis was to help prevent this funky hf vertical not be picking up any close shack-noise, or have a random-wire pattern.

But please don't cut up your existing feedline for this test!

 

[cmdrwill]

But please don't cut up your existing feedline for this test!

At least not yet.

 

[nanZor]

Heh, for sure - run a test jumper choked off first. And when I think about it, this may not be practical if your feedline is clamped down your mast, as it will couple into that rendering the whole "controlled common mode" hf reception trick moot.

Shoot - wish I had thought of that or I wouldn't have asked. Now to do that means isolating at least part of the feedline away from the mast... arggh.