[RFI] Inter-station interference question.
Mike / W5JR
w5jr.lists at gmail.com
Thu Nov 21 13:08:48 EST 2019
Hi, Martin. David/K1TTT also have very valid points about signal egress and ingress among various components in the RF path. The signals sometimes are a challenge to contain them to the signal path.
As for shields over a foil braid, two different situations can occur. One is a broadband noise that raises the noise floor generally, which is not what you are currently describing. Any movement of the braid/foil cables probably will generate this noise when a transmitter is active, especially if it’s in the RF path. Initially, this condition may not be present, but over time, it’s very likely to appear.
The braid against the foil oxidizes and creates the non-linear junctions you are familiar with from your VHF experiences. If not a broadband noise, it can be a source for generating rich harmonics external to the intended suppression techniques you are installing.
Also, any “loose” antenna parts are a prime source of PIM. All metal to metal connections need to be tight and a anti-oxidation grease applied to minimize the PIM potential.
I feel your pain about adding a second transmitter to what “appeared” to be a perfectly working single transmitter station. In the Georgia QSO Party for the past 5 years, we have been running a Multi-2 Rover station while in motion for 900 miles across 20 hours over 2 days. The two mobile antennas are 5 feet apart. I’ve bonded every body part and purpose built two stout frame antenna attachment pods. Even still, when we go over a rough bump, there will be a blip of broadband noise from some vehicle metal to metal junction. We have found several HF mobile antennas that work great by themselves, but are significant broadband noise generators in a Multi-2 situation.
Keep hammering away at it. You are on the right track.
Mike / W5JR
On Nov 21, 2019, at 6:00 AM, Martin Sole <hs0zed at gmail.com> wrote:
Dear Jim and Mike,
Thanks very much for the prompt replies.
Jim, I think the chokes are either from your latest work using big 31 rings or from the design by GM3SEK using multiple big beads and with I believe at least two such chokes in series. That is for each of the feeds to the antennas. The chokes are placed just slightly before the Array solution balun, again one at each feed point. I will review your other document as well, I can see it references issues similar to that which I have observed first hand. Yes the harmonic DOES have a growl to it!
The cable is predominantly 1/2" heliax of various manufacturer, based on what is available in country but all used in pro installations so generally decent stuff. There is some RG214 and some may have a foil. I do know that some of the stubs and their associated coupling cables I prepared are made with an RG214 that has 2 braids plus a foil and a dielectric that has 0.8v.f.
If foil and braid coax is used what is the issue?
Is there a PIM risk between the braid and the foil?
I am familiar with PIM having seen it first hand at VHF though it was always true intermod, being two incident signals creating a third order product. Though in those cases we never considered or cared about harmonics, for other reasons, I can agree that the non linear device should indeed produce all possible products including harmonics of a single source.
Regards es tnx
> On 21/11/2019 12:40, Mike - W5JR wrote:
> If any of the “RG214” is in fact a shield over foil braid, replace it with real RG214. As Jim mentions, passive intermod (PIM) usually rears its ugly head after installation of all the cool stuff. You will want to confirm that all of the components, radios, amplifiers, filters, feedlines, towers, guy systems, are *bonded* together to further reduce the possibility of PIM.
> PIM will be the biggest hurdle in making any multi transmitter situation play nice. Often the PIM generator is external to your feedline or antenna. It’s the “killjoy” of many a Field Day setup over here.
> Mike / W5JR
> Alpharetta GA
> On 21/11/2019 11:52, Jim Brown wrote:
> Hi Martin,
> here's an "overview" talk that I've done at Visalia last spring and for a couple of clubs, including NCCC, this fall.
> Among other things, check out the chokes in my newest Cookbook, which are significantly better and more repeatable designs than what I was able to do 10 years earlier.
> Also, consider passive intermod.
> 73, Jim K9YC
>> On 11/20/2019 8:22 PM, Martin Sole wrote:
>> This is about a two radio inter-station interference problem. A bit different perhaps but still RFI of a sort
>> Looking for some input to help me understand a bit better what is happening and maybe correct some errors in my thinking.
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