Topband: Placement of RX feedline chokes

[Jim Brown]

On Wed,2/3/2016 10:10 AM, Herbert Schoenbohm wrote:
> It has come to my attention by a seasoned topbander that I have this 
> all wrong and the decoupling coil should be close as possible to the 
> Beverage transformer.  He reasoned that the long RG6 cable (some 500 
> feet long)  couple random pickup right back into the Beverage 
> transformer and could also reduce the side lobe rejection from the 
> Beverage itself. So where is the best placement for for a feed-line 
> choke? 

He's right -- if you're using only one feedline choke, it should be as 
close as practical to the feedpoint (that is, out by the transformer, or 
up in the air for a TX antenna). There is also benefit to one or more 
additional chokes on a feedline. Two benefits. First, it minimizes 
interaction between that feedline and other antennas, fulfilling the 
same function as egg insulators breaking up guy wires.

Second, it minimizes feedline current, which, due to imperfections in 
the coax shield, can convert noise picked up in the shield to a 
differential signal on the coax. The metric for this conversion is 
called the transfer impedance of the cable, and is defined as the 
differential voltage divided by the shield current. EMC guru Henry Ott 
notes that the lower limit for transfer impedance is the resistance of 
the shield at the frequency of interest. The uniformity and the 
"coverage" of the shield also contributes.

Coax made for CATV is optimized for its performance at VHF-UHF. Shields 
are typically thin Al braid plus one or more layers of Al foil. These 
are pretty good shields at VHF/UHF, but their relatively high resistance 
can seriously degrade shielding at MF and HF.

73, Jim K9YC