Re: [TowerTalk] 12V through coax to switch antennas remotely

[Grant Saviers <grants2@pacbell.net>]

At some frequency above SRF the inductor starts to behave more as a 
capacitor due to its parasitic capacitance.  I use a VNWA to measure 
SRF  which has some pitfalls (or the Bourns spec) but for this 
application is close enough.  I think in the old days a grid dip meter 
loosely coupled would find it close enough.  Several other techniques 
and instruments also work, google it.
https://www.analogictips.com/when-inductors-self-resonate/

Grant KZ1W

On 4/3/2018 10:45 AM, N2TK, Tony wrote:
> Grant,
> How do you measure for self-resonance and how important is it for what I am
> doing?
> N2TK, Tony
>
> -----Original Message-----
> From: Grant Saviers [mailto:grants2@pacbell.net]
> Sent: Tuesday, April 03, 2018 11:59 AM
> To: N2TK, Tony <tony.kaz@verizon.net>; towertalk@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
> SRF.
>
> 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
> above?
> > 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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