[FARC] Kirk's antenna gremlins

[kirktal7237 at msn.com]

--------------------------------------------------
From: "David Matthews" <dave at djmatthews.com>
Sent: Thursday, February 14, 2008 9:22 PM
To: "Frederick, Maryland ARC" <farc at mailman.qth.net>
Subject: Re: [FARC] Kirk's antenna gremlins

> 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.

Yep I know a club member that uses a 40 meter vertically oriented loop on 
one other band for which he calculated the multiples, but in no way does he 
use it on all the bands.   Why?  Because he doesn't believe in tuners 
(because they don't fix the problem) and runs a KW via an amp which would 
surely damage the radio or possibly the antenna.  He's a loop zealot and 
also has a 17 meter vertically oriented loop, no other dipoles or beams.
>
>
> 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'd like to get a dipole to work first.
>
>> ...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.

Well that's the rub, "less-than-perfect way to get the job done."   The job 
is not getting done, at least not to a standard that most experienced hams 
will expect.    But I have to agree with you, I have made DX contacts on 20 
and 17 meters with an 80 meter dipole.  And most DX contacts report 599 
signals, even though they have to ask for your name and callsign 3 or 4 
times.  So that tells me something about how I'm getting out.

>> ...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.

Yes, but when you're trying to enter the interstate in front of speeding 
tractor trailers it gets downright scary.  Enter a ham radio contest with 
anything less than a Porsche and you'll be disappointed and irritate all the 
other contesters to boot.
>
>
>> 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.
>
I'm way out of my element here but reactance is the term for capacitive and 
inductive resistances which restrict the flow of AC current, right?   These 
resistances come from non-inductive or non-capacitive materials like wire. 
The value of reactance in an inductor or capacitor varies with frequency. 
In a circuit when the frequency is higher inductive reactance is higher and 
capacitive reactance is lower.  When the frequency is lower, inductive 
reactance is lower and capacitive reactance is higher.  The term for both 
capacitive and inductive reactances and their interaction in a circuit is 
called impedance.  Where these reactances are equal is called exact 
resonance.   Do I have it right so far?

Now it is only at a certain frequency where exact resonance will occur, at 
any other frequency the reactances are not equal and the circuit will either 
become more inductive or more capacitive at the antenna feedpoint.   This is 
an unbalanced condition which results in a difference in impedance measured 
in ohms between the antenna and a 50 or 75 ohm coax feedline.    In fact the 
impedance on a dipole, depending on the value of the reactances at various 
frequencies could be several thousand ohms.  Am I still on track?   If the 
value of the reactances are great, the unbalanced condition results in a 
high SWR and a large power loss.

A tuner fixes the unbalanced condition (impedance mismatch) as far as the 
radio is concerned but it doesn't change anything at the antenna, you'll 
still have an unbalanced condition with a high SWR and a large power loss if 
the reactances are high.   There's an old ham saying I've heard several 
people use that goes like this and I'm sure you've heard it.   "You can load 
up or tune a paper clip, but it doesn't mean you're getting out."   SWR 
being a ratio of reflected power to forward power;  X watts up, Y watts 
reflected down, X minus Y  watts out.  Does it work this way?  If so, add 
feedline length and type and the losses associated with that, bad 
propagation, high QRM and you've lost even more.    I've read it takes an 
SWR of 3.7:1 to double your DB losses.  In some of my tests I figured I had 
closer to 10:1 SWR on all frequencies other than 80 meters so this is 
probably the reason I have trouble in making contacts on the other bands

> 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.

I was talking about apples and this is an orange.


>
>
> 73 de K3MV
>
>
>
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