Topband: Placement of RX feedline chokes

[Herbert Schoenbohm]

Interesting information Jim because my quietest Beverage is fed with 
some double shielded double foil (braid foil braid foil) with robust 
braid RG-6 type cable but in order to get it to fit a connector both 
braids are tied together.

Herb, KV4FZ


On 2/3/2016 6:59 PM, Jim Brown wrote:
> 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
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband