HF ANTENNA TUNER
by Harry Lythall - SM0VPO
Introduction
I keep getting E-mail asking me for an antenna tuner (ATU or ASTU)
for the HF bands. First of all let me say that I do not believe in
them, although I have built a few for one reason or another. But I
am quite willing to share my experiences with you too.
The purpose of an ATU (Antenna Tuning Unit) is to match the antenna
to the receiver or transmitter. The mis-match most frequently occurs
when the system is not properly built. In order to improve the match
between (say) a vertical 1/4-wave ground-plane antenna and a 50-ohm
transmitter one should really make adjustments to the antenna until
the system is matched (remove the errors). In practice we can add and
ATU or ASTU (Antenna System Tuning Unit) and have a few more nice
pretty knobs to show visitors. It works on the principle that two
wrongs DO make a right (add an equal and opposite error). The ASCTU
(Antenna System Correction & Tuning Unit)however is 100% passive, so
it must by definition have a loss. But let us build one anyway.
Simple Matching
A simple 1/4-wave antenna that is too short (or too long) will behave
capacitively (or inductively). Simply adding inductance (or capacitance)
at the bottom of the antenna will correct the problem and make
it present a resistive impedance to the transmitter. Ok, now someone
is now going to send me E-mail saying:
I have a 6'2" aluminium pole, 2 Mars-bars diameter,
3-meters off the ground, fed with 0.75-furlongs of URM76. What size of coil
do I need? Please state wire guage, coil diameter, turns spacing, length
of wire needed and it's approximate weight to make it resonate at 1.4MHz.
Please also attach a photograph of the finished coil to the reply.
The answer is probably somewhere between
1 and 1000 turns, dependant upon the relative humidity (not stated!).
I used this inductor method with my multi-band HF Balcony
Antenna project. The main dissadvantage of this technique is that
there is no visible unit in the shack with loads of knobs and "sort
of pushy-things". Not very impressive!
Adding Knobs
If you want to match a transmitter
to a random-length of wire then the situation can become a little more
extreme and requires a little more than a coil of wire. When I first
began transmitting (as a pirate on 6.5MHz) I wound 20 turns of wire on
a toilet roll holder and connected this between the transmitter and the
antenna. A variable capacitor either end of the coil to ground enabled me
to tune and load the antenna.
It must be stressed that I have had some bad experiences using those
cheap plastic tuning capacitors you get out of transistor radios. They
work Ok in this application, providing you do not apply more than about
one watt of power. Apply any more and you will have a nice pyrotechnic
display. Use air-spaced capacitors for tuning.
Tuning Procedure
- Set up yourt transmitter to deliver about one watt continuous into a
dummy load, via an SWR bridge.
- Set the LOAD capacitor (right-hand) to mid position.
- Adjust the TUNE (left-hand) capacitor for a dip in reflected power.
Note the reading.
- Advance the TUNE capacitor a little and repeat 3 above.
- If the new reading is higher (worse) then reduce the TUNE and repeat 3 above.
- If the new reading is lower (better) then advance the TUNE and repeat 3 above.
- Repeat 5, 6 and 3 over and over again until the SWR becomes 1:1.
Mark the setting for each band/antenna. It is quite easy after a little
bit of practice (just like playing the glockenspiel with your left hand
after drinking half a bottle of vodka).
Multi-band ATU
Right, so you want to cover the whole HF band? Then you need to change
the size of the coil. Wind a coil on another "bog-roll" center using about
40 turns of "thick" wire (down to about 3.5MHz). Go up to about 120 turns
if you want to cover down to the top end of Medium Wave. Now make a few
tappings and connect them with a rotary switch to short-out all but (for
example) 3, 7, 12, 20, 50 and 120 turns of the coil. This is the circuit
of what we need:
Coil Winding
The coil should be wound using insulated (enamelled) wire. If you want
to make a decent permanent job of it then wind on a bit of plastic conduit
tube about 3cm Dia. Do not use black tube, this often contains carbon. You
can easily make connections to the turns by applying masking tape and using
a scalpel blade, cut away tape from one turn. Scrape the enamel off the
exposed turn and solder the connection. That sounds complicated so here is
a picture (from another project) of the technique:
With this switchable coil you will be able to match a random length wire
antenna over most of the HF band. The tuning procedure is exactly as given
above but select the switch position that gives you the best match.
Ideal Solutions
The ideal solution is not to have an antenna that needs an ATU is the first
place. But since you are reading this then you obviously need one. A transmitter
with a 50-ohm output impedance will be perfectly matched to a 50-ohm feeder
but it is the antenna that is at fault. Ideally the ATU should be placed not in
the shack but at the base of the antenna!
Not really practical. Visitors to your shack cannot be impressed by it and
it is difficult to adjust sitting at the top of a 30-meter tower. Normally
the ATU is in the shack and the feeder then becomes a part of the miss-match
that is being corrected. Anyway, I have now given you a start for experimenting.
Unfortunately I cannot give you the box dimensions, overall project weight or
even show a photo of the finished project. I have never built one of these
instruments in a form that I would want to show off. I have built several,
but only as a temporary measure.
That's it! I have nothing else that I want to tell you!
Have fun with this project. Regards from Harry - SM0VPO
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