Topband: Common mode choking of beverages - which side of grounded shack entry panel?

[donovanf at erols.com]

Hi Mark, 


It can be very challenging to reduce common mode adequately to preserve 
the directivity of low signal level antennas such as the YCCC 9 circle. 
My preferred approach is to bury all cables interconnecting the amplifiers 
at the base of each vertical and the phasing combiner/controller, and the 
cables running from the shack to the phasing combiner/controller 


As a minimum you should use the nine common mode chokes strongly 
recommended in the YCCC 9 circle array users manual. 


YMMV if you're QTH or nearby locations produces high level RFI. 
My preferred approach is to kill all RFI at its source. 


A peripherally related issue: Running DC power through the same coax cable 
as low level RF signals is asking for trouble. There's a high risk of RFI 
generating micro-arcing if any trace of water or moisture enters the connectors. 
If you must combine DC and RF into the same coax cable, you must 
to stuff your connectors with silicone grease to eliminate all air gaps. Water 
very easily migrates down the threads of CATV F-type connectors unless 
the threads are also protected from water migration. 


73 
Frank 
W3LPL 


----- Original Message -----

From: "Mark n2qt" <n2qt.va at gmail.com> 
To: donovanf at erols.com 
Cc: topband at contesting.com 
Sent: Sunday, October 4, 2020 8:18:20 PM 
Subject: Re: Topband: Common mode choking of beverages - which side of grounded shack entry panel? 

I’ve heard that on low signal strength receive arrays, having multiple grounding points, 
for the coax shield, such as for lightning protection at the house entrance as well as at 
the array itself and its associated antennas can be a potential source of induced noise. 

Is this true, and does this imply a need for multiple chokes on the feed line and control 
cables? And where should they be located? 

(In my case this is for a YCCC 9 circle, but should apply to other systems as well) 

Mark. N2QT 

> On Oct 4, 2020, at 2:24 AM, donovanf at erols.com wrote: 
> 
> Hi Bill, 
> 
> 
> The antenna feed point is where common mode signals on the outside 
> of coaxial cable shield most easily and severely degrade antenna 
> directivity. This a critical antenna performance issue for very low 
> signal level compact receiving antennas such as small loops and 
> short high impedance verticals and somewhat less critical for higher 
> signal level receiving antennas such as Beverages and resonant 
> short verticals (such as umbrella verticals). 
> 
> 
> Common mode signals less severely affect the performance of 
> high signal level antennas such as Yagis, horizontal dipoles and 
> quarter wavelength verticals over extensive radial systems 
> laid directly on the ground. However, multi-transmitter and 
> SO2R contest stations should use common mode chokes at their 
> antenna feed points to minimize cross-station interference. 
> 
> Double shielded coaxial cable or hard line coax with properly installed 
> high quality coaxial connectors greatly minimize the penetration 
> of common mode signals into the coaxial cable and the equipment to 
> which it is connected. In most cases common mode chokes will not 
> be needed in or near your shack. 
> 
> 
> If common mode signals are penetrating into your transceiver 
> -- especially in a multi-transmitter contest station -- the most 
> effective common mode choke location is directly at the coaxial 
> cable connection to the transceiver. 
> 
> 
> 73 
> Frank 
> W3LPL 
> 
> ----- Original Message ----- 
> 
> From: "Bill Conwell" <bill at conwellpdx.com> 
> To: topband at contesting.com 
> Sent: Sunday, October 4, 2020 5:24:05 AM 
> Subject: Topband: Common mode choking of beverages - which side of grounded shack entry panel? 
> 
> Sorry if this has been asked and answered previously, but I don’t find it 
> in a check of the archives. 
> 
> 
> 
> I’ll soon have an aluminum entry panel in my shack for all entering TX and 
> RX lines (as well as rotator lines and misc control cables). The panel is 
> copper-strapped to ground rods fifteen feet away, and to station ground. 
> 
> 
> 
> My beverages have common mode chokes near the antennas, followed by 250-500 
> ft of RG6 coax to the entry panel. (TX antennas are also choked, near 
> their feedpoints.) 
> 
> 
> 
> If I were to add further common mode choking at the shack end of the 
> beverage coaxes, where should such choking best be applied – to the RG6 
> cables on the outside, or the shack side, of the entry panel? 
> 
> 
> 
> 
> 
> Tnx, 
> 
> 
> 
> /Bill, K2PO 
> 
> Portland, Oregon 
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