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.
Mark GM4ISM
Sent: Thursday, June 28, 2018 8:32 PM
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.
