Topband: Palomar R-X Noise Bridge

[Carl]

----- Original Message ----- 
From: "Tom W8JI" <w8ji at w8ji.com>
To: "'TopBand'" <topband at contesting.com>
Sent: Saturday, February 15, 2014 12:08 PM
Subject: Re: Topband: Palomar R-X Noise Bridge


>> The lowest loss cable I have here is 75 Ohm 1" General Cable Fused Disc; 
>> its under a differnt name these days. Mostly air with poly discs and used 
>> for the 200' runs for 10M, 2M, and 222 MHz.
>>
>> For the 160/80 inverted vee it is 450' of regular foamed 3/4" 75 Ohm CATV 
>> hardline with a RG-11 jumper and plenty of ferrite to the feed point. Ive 
>> been using ferrite sleeve baluns since the mid 70's; I was introduced to 
>> them by the company I worked for who was building equipment for the joint 
>> CIA/DOD Tempest program.
>
> The lowest loss cables have large, smooth conductors that are the maximum 
> possible size for the cable impedance. Dielectric is largely meaningless, 
> except as it might affect conductor size.

** Which directly affects loss. While meaningless at 160 even in long runs 
the difference between the 1" Fused Disc and 1" solid foam Commscope is .5 
vs .65dB/100' at 222 MHz which can add up to no contact in long lengths.


>
> We can argue this point endlessly, but it will always come back to the 
> conductors.

** As close to an air dielectric as possible will have the largest diameter 
center conductor and lowest loss. Adding any other dielectric requires a 
smaller conductor to maintain the same impedance and with its own extra loss 
caused by the dielectric choice so it will always be a contributing factor 
since its dielectric constant and capacitance per foot varies and is not a 
lossless medium. It is also frequency sensitive.
A nitrogen pressurized coax is about as good as it gets.


The exception would be some horrible dielectric or operation way
> up above normal VHF/UHF with marginal dielectrics.
>
> It is the way it is. The confusion probably occurs because dielectrics 
> with more air allow a larger conductor to be used for a given cable 
> diameter and impedance, it is not because the dielectric has less loss.

** Im not confused but for the sake of the forum it is not exactly related 
to 160 so lets leave it for elsewhere.

Carl
KM1H