[TowerTalk] Antenna Measurements & the Mosley ad.

[Guy Olinger, K2AV]

Hi, all.

I suppose that all of the material that could be abstractly related to this
comparison would be a book. Practically, though, do they actually bear on
the case in point, with a large enough effect to do damage? In this case I
think not, particularly with a distant, unchanging receiving setup.

While I acknowledge the power of these considerations in various situations,
these were *all* yagis on the same test platform, actually of very similar
design, shape, and size, if you include quads, g5rv's, loops, verticals,
etc, in your universe of antennas.  I grant your concerns in a heartbeat to
compare a yagi at height, say, with an array of 1/4 wave verticals.

> What about a condition where an antenna does not have a perfectly
> horizontal radiation pattern compared to another antenna, say from
> feedline radiation or re-radiation from clutter in, near or around the
> test site?
>
> A problem with accuracy could easily occur, since the horizontally
> polarized component is greatly attenuated over the same distance
> compared to any vertical component. The slightest amount of
> pattern tilt could greatly affect path attenuation.

That presumes that the actual horizontal null is as deep as the theoretical
one. Here the ground clutter and irregularity, as long as it isn't moving,
tends to IMPROVE the accuracy of the test, since it takes a LOT more
off-angle radiation to skew the result. And it's not like it's hard to level
out a tribander against the horizon. And the dipole measurement is common to
all the tribander figures, and though it might affect absolute gain, would
not figure in the comparison.

>That's why it is
> imperative, in a test range, that the antennas NEVER be tested
> where site attenuation is high and/or where re-radiation could
> greatly affect results.
>
> If you want to see an example where this occurred, there is a web-
> site that proclaims by removing an antenna tuner and using a stub
> match, several S units were gain in field strength. The test protocol
> was a groundwave path over a few miles on 80 meters.

Depending on how grievously mismatched the antenna was, the result may have
true for DX as well. Most of his power could have been radiating in
directions he didn't care about, at the moon, or heating the sod. Did his
contest scores go up after he made the change?

I would also ask the question (though impolite to do so) whether the
receiving end had the ability to distinguish 6db and 20 db, given the
particulars.

> By simply removing an antenna tuner and feeding the feedline
> direct through a stub, the person providing that data is totally
> convinced he gained twenty dB or more. Let's assume the stub
> was 100 percent efficient. In that case the tuner would have been
> 1% efficient. Since he ran a kilowatt through the tuner, it would
> have had to dissipate 999 watts as heat without failing.

No disagreement whatsoever on the merits and particulars of the above,
regardless of where I think the power was really going. But does it
substantially apply to a COMPARATIVE test of extremely similar antennas on a
common test jig?

> The true data above is a shining example of why a cluttered
> groundwave path with high attenuation should never be used to
> determine efficiency or gain.

You mean ABSOLUTE efficiency or gain. They were COMPARING apples with apples
on the same test jig.

> Testing a horizontally polarized antenna close to ground over a
> large distance is always a bad idea, even in a controlled test range.

Except, Tom, this is EXACTLY the case we all have, unless you are on a 1000
foot tower, talking to another ham on another 1000 foot tower (well, maybe
on W4AN's mountain, but that doesn't count...). The intent is to test the
antenna how we use it. I can live with that and agree to never mention the
gain as an absolute figure.

> The larger the distance and the more cluttered the path and area
> around the antennas becomes, the more unreliable the test
> becomes (even in an A-B test), unless the antennas all have the
> same exact pattern (including the reference antenna).

Actually, in the relevant *direction* of maximum gain, say 0 to 2 degrees
vertical, plus and minus 2 degrees horizontal, (though I don't have a TA
chart on the test site -- Steve??) they DO all have the same natural pattern
SHAPE including the dipole, almost no divergence from a spherical front.

> This isn't to say the results aren't valid, just that the test was far
> from ideal. The results would be much more reliable if the test was
> made at 500 feet distant, rather than 5000 feet or more. Especially
> if the close spaced test was made in an open location.
>
> > 2) The modeling presumes the ground is a perfect plane with uniform
> > electrical characteristics. ANY departure from that and the ground
effect
> > gets ill-defined, losing its sharp horizon null. It is almost never as
> > deep a null as predicted. You can partly model real ground using the
> > program TA, which allows the entry of height variations and their
> > distance. With just a little messing with the ground countour, the
ground
> > null can be significantly reduced (ground sloping down from the antenna
> > for five to ten times the antenna height, for instance). The program
> > cannot, however, add in the variation in the electrical characteristics
of
> > the ground which would further difuse the null. In any event it is a FAR
> > FIELD effect, and if the tested antennas are all set at the same height
> > over the same spot, will NOT contribute any false divergence for the
test
> > results.
>
> It is not only a far field effect, it is also a near-field effect. The
> directivity of each antenna can greatly affect the results, especially
> in a cluttered test site, when scattering or re-radiation occurs. With
> that in mind, what you say above is true ONLY if the antennas all
> have the same pattern.
>
> Let me give a specific example:
>
> Let's assume we are comparing a dipole to a yagi for the purpose
> of evaluating gain. Let's also assume something near the antenna
> under test but NOT in the direction of the receiver is re-radiating
> energy that re-enforces energy in the direction of the receiver.
>
> A bidirectional antenna with a wide pattern, like the reference
> dipole, would couple to the re-radiator much differently than a
> directional antenna under test. The reference dipole could easily
> have gain that does not show up at the receive site as a FS change
> when the dipole is rotated, and that gain would not be available to
> the directional antennas.
>
> Let's also assume an object, let's say a telephone wire near the
> antenna under test, re-radiates and skews the pattern off
> horizontal. Under this condition an antenna under test that radiates
> with a slightly different pattern than another, but basically the same
> gain, could easily produce a stronger or weaker signal at the
> distant receiver depending on how it couples to that near-field (or
> far-field) re-radiator.
>
> A cluttered test site should NEVER be used for gain measurement
> meant to apply at different angles or locations where the results are
> plotted, especially when the pattern of the reference antenna is
> greatly different than the antenna being tested. I can't think of a
> single test in the world that would ever be considered a valid gain
> test when gain is measured OUTSIDE the main lobe of the
> antenna, or on the slope of the main lobe, unless it is gain only in
> that direction and angle that the tester is concerned with.

Yes, again if you speak of absolute gain. They were publishing relative
gains of very similar antennas. It was true they were expressed in units of
gain over a dipole at that site. But why not? Interestingly, the gains
approach but don't quite make it to the modelling numbers. This would be a
positive reflection on the tests.

>
> Which leads to another question...how perfect was the measured
> pattern of the dipole in the actual test site environment?
>
> The only way to answer this question would be to move around the
> dipole and plot the pattern near the dipole. If the site suffers from re-
> radiation or scattering, rotating the dipole and measuring FS at a
> distance location might never show the problem, although if the
> dipole pattern is not perfect when measured at the distant site we
> could be certain the site is polluted.
>
> > At one time, Mosley may have been the best in the field. That was then.
> > Have they been setting pat on the old designs? Metal covered traps?
> > Perhaps more durable, but you won't ever catch me winding a coil inside
or
> > around a metal pipe. It's lossy. There is induced circulation current.
> > It's as if I added a slightly resistive closed loop coupled to the coil.
>
> All traps, and all linear loading sections, add loss. At least with a
> trap the loss is primarily confined to the conductors inside the trap
> so we can get an idea about the loss from temperature rise, or
> easily measure the traps. With linear loading, we have the same
> losses distributed over a large area. Losses can be masked
> because heat transfer is increased.

My comments were not to tout LL over traps. LL has it's own set of problems.
In my own mind it's one manufacturer's traps versus another's, versus no
traps or fewer traps for the same number of active elements. In 1965, given
what we know we didn't know then and the lack of computer modelling, I think
the small tribanders were remarkable. I had a TH-3 and loved it. Wouldn't
put one up now, but I loved it then and make no apologies for it.

> The bulk of the loss occurs primarily for two reasons:
>
> 1.) Anything that adds capacitance across an inductor increases
> the circulating current in the inductor (or stub).
>
> 2.) A "shorted turn effect" will reduce mutual coupling from turn to
> turn and also generate eddy currents that reduce coil inductance.
> That means more conductor area with current flowing is required to
> produce the same inductance.
>
> The result of this is that the bulk of heat is in the coil, and not in
> the shield. Even so, the small amount of heat generated in the
> shield by eddy currents still contributes to trap heating. What this
> means is virtually all of the power lost in the trap heats the trap
> internally, and most of that heat is right next to the plastic form.
>
> Failures in Delrin tuner rollers occur when as little as 40-80 watts of
> power are lost in the inductor. Where are the melted trap coil
> forms?
>
> If the TA-33 has a trap problem why is the gain low uniformly on all
> bands?  On 20 meters, the trap offers minimal loading effect. It
> goes into an entirely different mode on 15 and ten meters. Even if it
> has slightly high ESR on 20 meters, why does ruin twenty meters
> when it is located at a point where current is low?
>
> How can the plastic in the traps handle hundreds of watts of
> heating without failure? Has anyone measured a trap? If no one has
> measured a trap, how do we know the trap is lossy?
>
> One of the main reasons the tests were done, to my knowledge,
> was to establish a good test protocol so we know what we are
> buying. I think that is an excellent idea.

They made the tests to compare tribanders.

> While the test results may (or may not) have established triband
> "pecking order", they were a long way from establishing absolute
> gain.

Again, let me state that they were COMPARING tribanders. All the discussion
I have ever had with the parties involved indicates they have the same
misgivings about stating absolute gain as you, even if they do not elaborate
the theory in your usual precise textbook manner.

Mosely would not give a hoot about the test if the test's absolute gain
figure on their antenna was not embedded in a plain comparison with their
competitors.

>I hope this discussion results in improved measurement
> methods, rather than mud-slinging and name calling.

I think the nature of that ad deserves a little mud. I am suprised that the
ARRL printed it.

> 73, Tom W8JI
> w8ji@contesting.com
>

73, Guy.

NO affiliation with any of the parties. Equal opportunity argue-er.




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