[TowerTalk] 12V through coax to switch antennas remotely

N2TK, Tony tony.kaz at verizon.net
Tue Apr 3 13:45:44 EDT 2018

How do you measure for self-resonance and how important is it for what I am
N2TK, Tony

-----Original Message-----
From: Grant Saviers [mailto:grants2 at pacbell.net] 
Sent: Tuesday, April 03, 2018 11:59 AM
To: N2TK, Tony <tony.kaz at verizon.net>; towertalk at contesting.com
Subject: Re: [TowerTalk] 12V through coax to switch antennas remotely

I have the same interest for a 160/80m relay switchable receive antenna feed
design on RG6 and came to the following conclusions (probably better ideas
out there).

The inductor should have its self resonant frequency at or above the maximum
frequency to be passed on the coax.  I selected a Bourns
652-RLB0608 180uH with 235ma current rating and corresponding low R and spec
4MHz SRF on a ferrite bobbin.  I've seen what appear to be 100uH Miller
solenoid wound chokes in commercial designs but have never measured their

The capacitors should have a SRF well above the max frequency.   I chose 
0.1uF Kemet 1210 SMD size 500v rated chip caps for the LC block and the C
pass .  The 500v rating is for a big lightning/static/crosstalk safety 
factor.   (I can show you some leaded 0.1uf HV ceramics with a measured 
SRF below 2MHz, I gotta wonder how one can make them so bad).  SMD
multi-layer chip caps are really superior to any leaded ceramic.

Elsie showed the DC block yields a 2k or so impedance across RG6 on 160m and
twice that on 80m, so a few percent loss of signal which for a receive
antenna is negligible.

Everything in stock at Mouser, not yet delivered.

Grant KZ1W

On 4/3/2018 6:32 AM, N2TK, Tony wrote:
> Have a question about the values of the choke to keep RF from the 
> relay and the caps for coupling of the signal. In different products I 
> see quite a bit of different values of both components.
> For the coupling caps I see values from .01-.2uf. It would seem 
> increasing the value of the cap reduces the Xc. And for the choke I 
> see values of 100uh-2.5mh. And it seems increasing the value of the 
> choke also increases Xl.
> My application is for 160M and 80M only with receive antennas.
> My question - does it really make a difference over the values listed
> Do all the values offer enough Xc or Xl to not cause any problems?  Is 
> there really a recommended "good" value for the caps and coils? Even 
> 100uh offers 1K ohms at 1.8MHZ and .1uf is .8 ohms.
> Still learning.
> Tnx
> N2TK, Tony
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