[RFI] Inter-station interference question.

Martin Sole hs0zed at gmail.com
Thu Nov 21 06:00:28 EST 2019

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
Martin, HS0ZED

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.
> tnx
> 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.
> http://k9yc.com/Multi-Station.pdf
> 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:
>> Hi,
>> 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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