[FARC] Kirk's antenna gremlins

[David Matthews]

From: "Kirk Talbott" <KirkTal7237 at msn.com>
Subject: [FARC] HF Gremlins

>> ..Ok, it's an 80 meter dipole... The plot also showed the other HF bands, 40, 20, 17, 15, 12, and 10 meters had totally unacceptable SWR, with readings that were full-scale off the SWR meter in the radio.     
Hi Kirk -

I'm surprised that in the responses to this email so far, nobody has 
addressed the fundamental issue at work here.

If you drive your half-wave 80-meter dipole with RF at 40 meters, then 
the electrical length of this dipole is now a full wave.  It is an 
inherent characteristic of antennas that are a full wave (or any integer 
multiple of a full wave) that at the very center of the antenna, the 
impedance is theoretically infinite.  In practice, this will end up 
somewhere between 500 and 2000 ohms).  At 20 meters your antenna is 
about 2 full wavelengths and again, the center feedpoint has an 
extremely high impedance.  At 10 meters, its 4 full wavelengths long, 
and once again has an extremely high feedpoint impedance.

Ohms law tells us that as the impedance of any load approaches infinity, 
the current delivered to the load will approach zero, no matter how much 
voltage you apply.  If your antenna has an infinite feedpoint impedance, 
it is the same as open circuit... nearly the same not being there at all. 

Tuners can effectively match things up to about 3:1.... and after that 
point, even if they can make the match, they get lossy.  Transmission 
lines don't have too much loss at low SWR's , but the loss goes up 
dramatically if you get yourself up into the 10:1 or 20:1 mismatch range 
that you' re dealing with here.  It is probable that on 40, 20, and 10, 
your feedline is doing most of the radiating (less the loss in both the 
tuner and the line).  When this sort of thing happens, you also 
typically get a lot of RF on anything connected to or near the rig.... 

There's really only one way to get an 80-meter half-wave dipole to be an 
acceptable match to a tuner at 40, 20 and 10.... and that is to feed the 
antenna off-center, so you are moving the feedpoint away from that point 
of infinite impedance at the center.   This is the principle used by 
Windom antennas....   You choose a feedpoint location away from the 
center, and try to pick a point where the impedance will be nearly the 
same regardless of which band you're operating on.... then you design a 
matching network to transform that impedance to match your feedline.  
For more information on this type of antenna see  
http://users.erols.com/k3mt/windom/windom.htm  I have used one of these 
K3MT-designed Windoms and can attest that it does provide an acceptable 
match on most bands, and without a tuner.

However, just getting power into the antenna is not the only objective.  
The other thing you need to be thinking about is the radiation pattern 
of the antenna:

     a) An 80 meter dipole driven on 80 meters will have the classic 
dipole radiation pattern with no radiation parallel to the wire, and 
maximum radiation perpendicular to the wire.

    b) If you were are able to sucessfuly drive that same wire on 40 
meters, the pattern would now be a 4-lobed cloverleaf.  Radiation would 
be maximum at 45 degree angles to the plane of the wire, and there would 
be zero radiation either parallel to the wire or perpendicular to the 
wire.  So that's four lobes and four nulls.

    c) carrying this principal all the way up to 10 meters, you would 
now have a theoretical horizontal plane antenna pattern with 16 lobes 
and 16 nulls (and it still would have nearly zero radiation either 
parallel or perpendicular to the wire).  In practice, ground reflections 
make things more complicated.    Sure, you'll be able to hit a lot of 
stations on field day.... but only the ones that don't happen to be 
within one of your 16 pattern nulls.
     
Tuner or not, whenever you try to drive an antenna on a band for which 
it was not designed, you will run into some compromise....   Perhaps the 
efficiency will be low.  Perhaps the bandwidth will be too narrow.  
Perhaps the radiated pattern will be less-than-useful.  Multi-band 
antenna design is really the art of choosing an acceptable set of 
compromises that you're willing to accept for your particular application.

If you're going to use a tuner to try to match a non-ideal antenna to 
your rig, one way you can dramatically improve things is by using an 
external tuner (either automatic or remotely controlled) that is at or 
near the feedpoint of the antenna.  Thus, most of your feedline will be 
looking into the nice clean 50 ohm resistive load provided by the 
tuner.... so the transmission line won't radiate and it won't suffer 
losses from SWR (and you'll keep stray RF out of the shack).  This will 
also provide you with a radiation pattern closer to what you expected.

If you want an external tuner to do the work, then the antenna length 
you'll end up choosing is some length which provides the least 
objectionable load impedance to the tuner at all of the frequencies you 
want to operate on.  Usually, this means you're going to select a 
compromise length that is not resonant on any of the ham bands... but 
the tuner will take care of that for you.   There are a number of pages 
on the web where people have set up spreadsheets to calculate what wire 
lengths are least objectionable to the tuner for the particular set of 
bands that you want the tuner to work with.    

But that's for an external tuner.   For an internal tuner built into the 
radio, the design objective is not to match the antenna to the 
transmission line, but rather, to provide a reasonable load to the final 
transistors so that:

    a) the finals don't get cooked by high current or high reflected voltage

    b) your rig will not go into ALC limiting (sometimes called VSWR 
foldback) to protect the finals, and will thus be able to deliver full 
power to the antenna jack

    c) the low-pass filter inside the rig will work properly because 
it's feeding the load impedance it was designed for... thus you're less 
likely to radiate splatter on frequencies you didn't intend to be on.

So although an internal tuner may be able to auto-tune a good match to 
the final amplifier, it can't do anything to improve a really bad 
mismatch between the feedline and the antenna (leading to low power in 
the antenna, high losses in the feedline and tuner, and various amusing 
effects in household electronic devices).
   

73 de Dave, K3MV