Topband: 12V through coax

donovanf at donovanf at
Sat Dec 7 18:02:50 EST 2019

Hi Tony, 

George's advice is good. A few additional points: 

- you should also install a 100-150 uH choke across the coaxial 
connector going to your radio, it will protect your radio from the 
DC voltage if the capacitor shorts out. 

- its very, very important that all of your connectors be well waterproofed. 
Any trace of moisture can cause micro-arcing with resulting elevated 
noise floor into your receiver. 

Good luck! 


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

From: "GEORGE WALLNER" <aa7jv at> 
To: "tony kaz" <tony.kaz at>, "tony.kaz--- via Topband" <topband at> 
Sent: Saturday, December 7, 2019 10:02:15 PM 
Subject: Re: Topband: 12V through coax 

The component values are not critical, especially in a RX only application. 
The rule of thumb I use is that the reactance (AC resistance) should be 20 x 
the impedance for the choke and 1/20 for the series capacitor. 
With 75 Ohms, that works out to 1500 Ohms, which is a 135 uH choke. Anything 
over 135 uH will do as long as the current does not saturate the choke. 
(Even 100 uH will be OK.) Don't use a too high value choke (over 3 mH) as 
its self resonance, or resonance with some external capacitance, may be 
within your bands of interest. Also, higher inductance chokes generally 
saturate at lower currents. 
The capacitor's reactance should be less than 3.5 Ohms. A 0.1 uF capacitor 
has a reactance of around 1 Ohm at 1.8 MHz, so should e more than enough. 
(But, I don't think you would notice any difference using a 0.01 uF 
capacitor (10 Ohms).) Make sure the capacitor's voltage rating is 
sufficient, including any kick-back from the relay (which could be over 100 
V peak). 
GL and 73, 

On Sat, 7 Dec 2019 15:59:17 -0500 
"tony.kaz--- via Topband" <topband at> wrote: 
> I am making up new boxes for my Pennants. I will be switching directions 
> with a relay in the box besides the terminating resistor and the choke. 
> My question is what are the correct values of the components for feeding the 
> 12V? I will be using this only on 160 and 80M. 
>For the coupling cap between the rig and antenna I have seen values from 
> .01-.1uf. For the choke I have seen values from 100 uhy - 4.7 mh. On the 12V 
> feed side I have seen values from .01 - .1 ufd. 
> Is there a rule of thumb for the range of correct values for 80 and 160M? 
> Thanks 
> N2TK, Tony 
> _________________ 
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