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LF: Re: Can I match 2 frequencies with one loading coil?

To: <[email protected]>
Subject: LF: Re: Can I match 2 frequencies with one loading coil?
From: "Mark" <[email protected]>
Date: Thu, 28 Jun 2018 23:27:16 +0100
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Reply-to: [email protected]
Sender: [email protected]
Hi All
The technique used for MF  broadcast systems to combine 2 or more transmitters into a single antenna is valid here.
This is not a ‘1 loading coil’ solution, but is relatively easy to implement as there is no interaction between legs.
Build a parallel tuned circuit for each frequency you want to use. These are referred to as rejectors.
The 2 rejectors are joined at the antenna driving point each leg goes to the  equipment.  ie the MF Rx  is passed through the LF rejector  to the antenna and vice versa
Each rejector will have a very high impedance at its resonant frequency but  have a  series reactance  that is manageable at the  pass freq.
You then add this series reactance at the pass frequency to the driving point impedance (it helps if you  can measure this and  consider in series reactance format)
The pass reactance is easily calculated for the rejector. You can then match each leg at the  wanted frequency with no significant  interaction between the  legs of  combiner / diplexer
Many broadcast antennas at MF are electrically short and often it is helpful to add a  pre-match  element at the driving point  a shunt C or series L or whatever  minimises the subsequent matching requirements on both legs. Matching is commonly done with Pi or T matching sections for ease of adjustment and improved harmonic rejection, although a simple L match would suffice on receive.
This technique allows transmitters at very high powers (50-100kW or so) on 2  or more  frequencies to use one antenna. It will work on receive just as well enabling a  proper 50 j0 ohm resistive  match ( if that is actually needed )  for n frequencies, you need n-1 rejectors in series  for each leg before the matching section
If you decide to try this on Transmit, watch out for the  increased ‘circulating currents’ in the rejectors due to the passed transmission. Currents and voltages in the C and L of a rejector are larger than the  expected current in the equivalent  pass reactance.
Sent: Thursday, June 28, 2018 8:32 PM
To: LineOne
Subject: Re: LF: Can I match 2 frequencies with one loading coil?
Try this, but you'll have to adjust iteratively:
Bands referred to as LF and MF
First make your LF loading coil so it resonates the antenna as it stands.    Now make an MF loading coil.  The problem is that t=when both are connected to the antenna teh lower value MF one shunts teh LF.  So add series C to the MF one to make a series tuned circuit, increasing the L to keep the system at resonance.   The net effect now, at LF will be an increased value of X shunting it.  As your additional C reduces and L increases, the residual at LF across the LF coil goes up.    Now adjust teh LF colil to bring it to resonance, then go back and redo the MF L or C.  Iterating between teh two until they both go.
I'ts a complicated setup, with two frequencies and three variables to play with - so an infinite set of values to end up with, but if I were doing it, I'd make up a spreadsheet and play around with values until a workable solution that resonated both antennas was found and looked OK.  Once resonated you can bring them to a match if needed.   But on Rx just resonating them ought to suffice
Or use a circuit analysis prog.    
On 28 June 2018 at 20:04, Chris Wilson <[email protected]> wrote:

Hello Mal,

Thanks Mal and Stefan, but Mal, I want to listen to and decode both bands
at the same time on 2 different receivers, sharing the one antenna. I
tried  shorting out the loading coil but LF reception is much poorer.
Not  sure how much of an effect no loading coil has at MF ,but suspect
a loading coil to resonate would be pretty helpful?

Thursday, June 28, 2018, 7:55:55 PM, you wrote:

> I use aircore loading and matching
> Use 2 coils and SW between them for desired band

Best regards,
Chris                            mailto:[email protected]

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