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Author Topic: antenna coupler help for medium waves?  (Read 10989 times)

Posts: 18

« on: October 16, 2009, 10:27:31 AM »

we have a university official radio station. the transmitter is about about 250watt with 50 ohm output at 1431khz medium wave band. my antenna is an inverted-L antenna with (5/16)*wavelenght total lenght (20 meters
vertical and 45 meters horizontal wire 2,5mm).  i have 3 scematics for antenna coupler system. The first and the second
schematic send me 2 radioamateurs. I calculate the third coupler with 4nec2 software. Which coupler is the best
for my antenna??? do you have to propose another schematic?  the transmitter is only for educational proposes.
i have a variable capacitor 500pf and a variable coil about 100uH. thanks

Posts: 0

« Reply #1 on: October 16, 2009, 12:16:10 PM »

The classic inverted L antenna is a quarter wave length of wire at the operation frequency that is in as much as possible a vertical orientation with the remainder being in a horizontal orientation.  It is basicly a ground mounted vertical with a bent top.

The Classic inverted L antenna does not require any kind of tune circuits at the feedpoint and can be easily fed with simple 50 ohm transmission coax.  Tune of the antenna is done by adjusting the length of the antenna to the operation frequency desired.

BUT the Inverted L antenna does require a good ground field to work against for any kind of efficiency.  I would suggest at the very least 32 ground radials each having a length of at least 1/4 wave length to 1/2 wave length long.  The greater the number of radials the lower the angle of radiation and the lower the feedpoint value.  Plus there will be less ground loss as the number of radials becomes greater.

The reason you seem to need a coupler is that your overall length of wire used in your Inverted L, if I read your post correctly, is longer than a quarter wave length which would of course mean that the antenna is only a random length wire in an Inverted L configuration and not a classical Inverted L.  Such an antenna would need some kind of tune circuit because the antenna itself is not resonant to the operation frequency.

This added length may give you a little extra gain on transmit but it comes at a price.  The coil an cap that you will need at the feedpoint to fake out your transmitter into seeing a 50 ohm antenna feedpoint  will limit the transmit bandwidth.

BUT THEN again, IF you are a commercial station restricted on only one frequency of operation you would not need a hell of a lot of bandwidth.  

As to circuits may I suggest you go to the nearest library and get a copy of the ARRL Antenna Book and look at the various antennas and how they are fed.  You will most likely find one antenna with a coil/cap that will be similar to your situation.

Posts: 18389

« Reply #2 on: October 16, 2009, 07:09:48 PM »

Actually, having your antenna 3/8 wave instead of 1/4 wave
will work better because it raises the feedpoint impedance
and makes it easier to match.

Either of the last two circuits (which are the same except
for component values) will work.  Have you measured the
exact input impedance of your antenna?  It is very difficult
to predict the exact value with a computer model because
it depends on the ground system, the effects of buildings
and the antenna support structures, etc.

Start by putting the capacitor in series with the antenna
and adjust it for minimum SWR.  You may find that is all
the tuning you need.  If the minimum SWR is still to high
because the input impedance is low (as it may be if you
have a good ground system) add the shunt coil and
readjust the capacitor for minimum SWR.  If the capacitor
isn't big enough you can add some additional fixed
capacitors in parallel with it, or us a small bit of
the coil in series with the capacitor - but only as
much as you need to get a match.

So the final result might look like this:  the feedline
is tapped to the coil at a point that depends on the
antenna impedance - perhaps 10 turns.  Then, if your
capacitor is a bit small, there is another tap on the
coil - perhaps 13 turns from the grounded end - that
goes to the variable capacitor.  The antenna wire
connects to the other side of the capacitor.  Hopefully
you don't need the extra 3 turns on the coil.

If you have measured the actual input impedance you can
calculate the required values (as it appears you have
done.)  If you have modeled the antenna you can still
do the calculations, but you will need to adjust the
values for best match.

Posts: 18

« Reply #3 on: October 17, 2009, 02:52:50 AM »

thank you for your answers firts of all.
WB6BYU the schematic that you propose is the follow?:

I would like to ask you, how many impedance must be the coil aproximmately for 1431khz, and if my variable capacitor is not enought, how much capacitance i must connect paraller to the variable capacitor??
if i understand, the 3 turns is optional.

Posts: 18389

« Reply #4 on: October 17, 2009, 08:48:15 AM »

The numbers of turns are only examples.   Either you must
measure the actual input impedance of your antenna and
calculate the coil and capacitor values from that, or
you have to put the coil and capacitor in place and
adjust them (by turning the capacitor or by tapping onto
a different turn of the coil) until the impedance is

There are too many variables (especially the ground loss)
to know for sure.  With high ground loss you will not need
the coil, but the antenna will be less efficient.  With
very good ground you will need fewer turns on the coil.
But it is impossible to know until you have measured the
actual impedance of your antenna.
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