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!
73
Frank
W3LPL
----- Original Message -----
From: "GEORGE WALLNER" <aa7jv@atlanticbb.net>
To: "tony kaz" <tony.kaz@verizon.net>, "tony.kaz--- via Topband"
<topband@contesting.com>
Sent: Saturday, December 7, 2019 10:02:15 PM
Subject: Re: Topband: 12V through coax
Tony,
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,
George,
AA7JV/C6AGU
On Sat, 7 Dec 2019 15:59:17 -0500
"tony.kaz--- via Topband" <topband@contesting.com> 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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