[FARC] Kirk's antenna gremlins

[David Matthews]

kirktal7237 at msn.com wrote:
> ....Unless you have acres and acres of property to erect a dipole for 
> every band, you kind of have to pick an operating frequency area and 
> stick with it and let all the other band capability of the radio go to 
> waste...

Not at all.... there are many ways to make a single antenna work 
acceptably (though not perfectly) on multiple bands.  You may have to 
try several different solutions before you arrive at something you 
like.  Such is the nature of ham radio.  Trying new stuff is just part 
of the fun.
 
> ...Do it any other way and you're subtracting performance from the 
> antenna/radio combination; add poor propagation conditions and like me 
> you'll have little success in making contacts...

Well, yes, we're at the lowest point in the sunspot cycle now, so things 
are more difficult... but far from impossible.  On Field Day 2007, I 
hooked up an Icom IC-703 (only 10 watts out on SSB) to an AH-4 external 
auto-tuner and spent a couple of hours experimenting with several 
different wire configurations from the 4th-floor balcony of my 
apartment.   The configuration that worked the best was a 27 foot 
horizontal loop formed into a triangle with the tuner at one of the 
apexes.   It loaded well on 40, 20, 15, 10, and 6 meters and I was able 
to make around 40 contacts in a couple of hours (some out on the west 
coast) despite competing with all of the QRM from other stations that 
had a lot more power and a lot better antennas.   Experimenting pays off.


In your case, it seems that 80 meters is the only place where your 
current antenna works decently.  But maybe the 80-meter band isn't where 
you should be starting if you're relatively new to this and perhaps 
getting a bit frustrated.  You've probably already discovered that 80 
meters is mostly a night-time band since during the day the D-layer of 
the ionosphere gets ionized by solar radiation and absorbs most of what 
goes into it.  You could try a different approach to 80 meters, just for 
fun.  Drop your dipole down really close to the ground (somewhere 
between about 3 feet and 10 feet).  At this height, the interaction with 
the ground means you'll be directing most of your signal nearly straight 
up.  At those high angles, you can work out to about 300 miles and you 
may find you get surprisingly good signal reports with not very much 
power.  This is NVIS ("Near Vertical Incidence Skywave") and some 
military HF stations depend on it.   It's fun - give it a try.  
  
> ...I understand a piece of wire cut to length for a specific 
> frequency, mounted at the wavelength height for that frequency, and 
> fed with a matched feedline will give the best results.    I know this 
> is a narrow (mono-frequency) viewpoint but like you indicated, 
> anything else is a compromise, another word for losing ground.   
> Multiband antenna?   What you've got here is mediocre performance on 
> all the bands.

"Compromise" is *not* a bad word if it means you've found a 
less-than-perfect way to get the job done.  If I have an antenna that's 
only half as efficient on a particular band, that means I'm down 6 dB 
from where I'd like to be.  But 6 dB is only 1 S-Unit on the other guy's 
receiver....  will he notice?  Probably not.   If you build multi-band 
antennas that use traps or loading coils or capacity hats, you're 
building in some losses... and in return you get something that works 
well on a practical level even if it doesn't come anywhere close to 
theoretical perfection.  

> ...Those guys with the multiband beams mounted on very tall towers 
> with rotators and using linear amps are ALWAYS HEARD with commercial 
> radio studio quality sound all over the world.  Must be something to 
> this.

Sure.  A super-charged Porsche would be a lot faster and more 
maneuverable than my 9 year old Saturn sedan... but its a fact that my 
Saturn works just fine for getting me wherever I need to go.  


> For 40 meters a half wavelength dipole is supposed to be 65 ft. long 
> at 7.200 Mhz. and 58 ft. 6 inches high.  If you shorten it or lower it 
> then it can't be resonant at 7.200 Mhz, can it? Multiband antenna 
> manufacturers supposedly get around the length issue for the various 
> bands with traps, but how do you get around the wavelength height 
> issue?  Mount it 200 ft. high and you're covered? 

It is fiction that an antenna must be self-resonant in order to radiate 
well.  For simple dipoles, self-resonance is simply a convenient 
condition because it means that a center feedpoint will not have 
reactances to tune out and the impedance will be a reasonably good match 
for coax.

Want an example of a good non-resonant antenna?  One of the most popular 
antenna types for 2 meters is a 5/8-wave vertical.   A theoretically 
perfect 1/4 wave "resonant" vertical over a perfect ground plane would 
have an impedance of 36 ohms.  A 5/8-wave is fairly close to 50 ohms 
resistance, but also has a capacitive reactance component that needs to 
be tuned out with a coil at the base of the antenna.  Because of its 
larger radiation aperture (and a resulting null at a high vertical 
angle), the 5/8 wave vertical will provide better gain toward the 
horizon when compared with a 1/4 wave or a dipole.   So that's just one 
of several concrete examples of where a non-resonant antenna can work 
just fine and maybe even have an advantage over a resonant antenna.


73 de K3MV