[FARC] Kirk's antenna gremlins

[kirktal7237 at msn.com]

The lack of response must be due to impatience with rookies.  Thanks for 
your response Dave, any and all help and advice I can get is welcomed though 
not always understood.

I've been in ham radio for a year now and the lesson I've learned from many 
hams is that there is no quick and easy answer to getting radios and 
antennas to work despite antenna/radio manufacturer and advertising claims 
to the contrary.  Try and cut a corner and you end up cutting performance. 
This is frustrating because we spend big bucks for multiband radios and 
there isn't an easy way to make use of all the bands.  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.

I've learned that the basic formula, 468/Freq in Mhz. gives you the length 
of your dipole and 421/Freq in Mhz. gives you the wavelength height.  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.

As for the technical aspects of HF antenna theory, that's light-years over 
my head, so far in fact that the study of it is painful.   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?  Has to be an oxymoron in reference to 
dipoles.  What you've got here is mediocre performance on all the bands.

Now I also know this from a year of listening to hams.  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.

73
KB3ONM
Kirk


--------------------------------------------------
From: "David Matthews" <dave at djmatthews.com>
Sent: Wednesday, February 13, 2008 9:03 PM
To: "FARC Reflector" <farc at mailman.qth.net>
Subject: [FARC] Kirk's antenna gremlins

> 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
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