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[TowerTalk] Antenna Measurements & the Mosley ad.

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Subject: [TowerTalk] Antenna Measurements & the Mosley ad.
From: Guy Olinger, K2AV" < (Guy Olinger, K2AV)
Date: Mon, 21 Jun 1999 23:56:50 -0400
Michael (W4EF) is a good representative of the commentary from one of the
major viewpoints about the tri-bander comparison.  Expresses himself and the
major complaints very well. I just don't agree too much, for this case.
Thank you, Michael, for your thoughtful post.  I think it helped me to get
my stuff together, though any one reader might disagree with that sentiment.
And now, to all of you in the loyal, respected opposition in this matter....

+AFs-We are talking about forward gain in this discussion. Not SWR, or F/B, or
F/S, or F/R (rear). Those are an ENTIRELY different matter. +AF0-

+AD4-    +AFs-W4EF+AF0- A 20dB null would imply off-angle radiation on par
+AD4-    with the direct ray. Small changes in relative phase and amplitude are
+AD4-    much more relevant when two strong signal are cancelling (out-of-phase)
+AD4-    than when they are adding (in-phase). Small differences in antenna
+AD4-    phase centers and sidelobe level that would have no effect on boresight
+AD4-    could be significant to nulled field strength.

+AFs-I have to give it to K7LXC that he can mount an antenna level. So I refuse
to consider horizon-challenged-tower-klutz-induced vertically polarized
components of enough strength to twist a null. Sorry.+AF0-

I can't say enough about the similarity of the antennas. The Mosley aside,
the difference between the worst and best antenna is an amount less than
what most people can discern in their ears. Although some people swear that
the decibel was invented so that 1 db was the median discernible (it's just
a convenient mathematical number from logarithms base 10), the Bell
Laboratories research showed that a gradual increase of 3 db was the point
where most people started discerning the change in levels. W8JI thinks the
antennas in the comparison are apples and oranges. I think we are comparing
bright red apples against dull red apples. Red apples all. The Mosley aside,
the largest distinguishing characteristic correlating to gain between the
antennas is boom length. The longer ones are stronger on average, the
shorter mildly less.

The only distinguishing differences? Traps or no traps or a combination,
exactly where and how long are the DIR's and reflector. (Personally I doubt
there's even a foot difference between the most forward and most rearward
effective center of radiation among the antennas.) Between Bencher and Force
12, one or the other will determine the absolute optimum settings of a
trapless 20 ft boom tribander, which will become the tribander equivalent of
the five-tube AM radio, a cheap, practical, effective design that buries all
previous designs until a new paradigm arises (transistors ate the five-tube
AM radio).

The null stuff, though TOTALLY significant if trying to measure out a SINGLE
antenna BY ITSELF, just does not matter in a comparative study of relatively
identical physical designs done sequentially on the same test jig. It's
going to be patterns and possible lossy designs.

+AD4-    +AFs-W4EF+AF0- If the measured HPBWs imply similar directivity and the
+AD4-    the deficit antenna is not bursting into flames on a regular basis,
+AD4-    then one has to wonder if other factors aren't at work.

Bursting into flames does not play. Let's take an extreme, assume 800 watts
of loss (+AD4- 50 percent, a half dummy--a full dummy load is not possible).
Constant CW? Never happens. Even in a contest, maybe 1/4 average duty cycle,
equivalent 200 watts continuous loss in the antenna. Worst case 15 meters.
Only 6 traps to dissipate the heat. Can a trap dissipate 33 watts? OK, even
if you say all the soup in the DE it's still only 100 watts But that can't
be, if the yagi has a pattern. Unless keydown endless RTTY, it would be hard
to make a case for a single trap dissipating more than an AVERAGE 50-70
watts.Think about most trap +ACI-flameouts+ACI-. Really water, insects, arcing
paths, and connections gone bad. If a trap can't dissipate 50-70 watts, then
every lamp in the country is a fire hazard.

In all my years, the only purely MELTED traps I ever saw outside 6-land were
lightning damage.

However, deliberately combine a mildly lossy trap with poor element design,
and you can unfortunately design into place what the study was reporting.


+AD4-    +AFs-W4EF+AF0- If the comparison was so good, then why don't the 
measured HPBWs
+AD4-    (half power beamwidths) correlate very well with the gain figures.

That expected correlation has a huge presumption about patterns behind it
that only plays absolutely in theoretical space, and almost plays only on a
nearly perfect antenna range. Does not apply in the regular world of odd
reflections. Especially if all the cries of +ACI-Null+ACE- Null+ACEAIg- are 
true.  But the
discorrelation would be equal for all antennas since they are on the same
single test jig. It's a comparison.

+AD4-    Efficiency?
+AD4-    As W8JI points out, large swings in efficiency on legal limit antennas
+AD4-    imply flaming traps.

NO. See above.

+AD4-     What about the clockwise versus CCW pattern asymmetry?
+AD4-    This would imply that second order effects are operating.

Or, that in one side direction we are getting some mild degree of reflection
off a hill. Straight on bore-sight hitting a null would would accentuate
that, but it would accentuate it for all of them, though mildly less for
sharper patterns. Do the larger gains have less asymmetry?


+AD4-    +AFs-W4EF+AF0- The case we have are skywave signals arriving over
+AD4-    a range of non zero elevation angles. This is why an tribander
+AD4-    at 50 feet works well on a fully open band, but gets trounced
+AD4-    by a yagi at 120 feet when the band is opening. It isn't that
+AD4-    the high antenna has that much more gain than the low antenna,
+AD4-    it just the angle were the high antenna has the gain. If you were
+AD4-    correct that near horizontal take off angles were most
+AD4-    representative of actual conditions, then I wouldn't have a pray
+AD4-    working Europe short path from the L.A. Basin which stares straight
+AD4-    at the San Gabriel mountains in that direction. In fact, I have no
+AD4-    trouble working Europe with a yagi at 80 feet, despite the fact that
+AD4-    it is staring straight at a 5000' mountain.

I agree. But what bearing does this have on a comparison of nearly
physically identical antennas being compared on the same test jig? That
there are somehow great differences in the shapes of the free-space patterns
forward plus/minus 45 degrees elevation, that would not show up looking
through the boom, that would account for all the Mosley drop?  Not. These
are all antennas designed to have their maximum free space gain staring
forward through the boom.


+AD4- +AFs-W4EF+AF0- If these antennas have maximum gain at zeroes degrees 
+AD4- then they are all rotating dummy loads as evidenced by the signals levels
+AD4- present at the receiver.

They DO have their maximum radiation forward. By the time it gets over there
the signal has experienced the usual low angle attenuation possibly some odd
multipath cancellation. So what. Suppose I elevated the receive setup to
2000 feet in a 100+ACU- non-metallic blimp, and then put a 20 db attenuator in
the line to my receiver. Would that invalidate the comparison, as long as
the attenuator was in line for all the antennas? I suspect some would
complain that there was no way to prove that the blimp didn't move.

They were not trying to come up with an absolute measurement of one antenna,
that could be compared with another absolute measurement of another antenna
made a year later somewhere else. They warn AGAINST extrapolating the
results of the comparison beyond the comparison. It's a comparison ONLY.


+AD4- +AFs-W4EF+AF0- The test data does include absolute signal levels which
+AD4- indicate that all the antennas (including the reference dipole)
+AD4- are significantly nulled from peak theoretical gain.

I would really worry about that if I was going to praise or flame an antenna
based upon a single test on a single antenna.  I don't worry if they are all
subject to the same conditions.

+AD4-The relative
+AD4- gains may approach the modeling numbers, but the absolute numbers
+AD4- relative to modeled gains indicate that all the antennas, including
+AD4- the reference dipole are rotating dummy loads.

Or that they weren't really trying for absolute numbers. Their philosophical
approach is to compare all antennas on the same test jig, so that those
considerations don't matter.


+AD4-    +AFs-W4EF+AF0- Of course. They are not in business to enhance our
+AD4-    understanding of antenna theory, they want to sell antennas. The
+AD4-    test results are direct threat to their business, so its only
+AD4-    natural for them to attack the test methodology or the motives
+AD4-    of the testers. I on the other hand, have no vested interest in
+AD4-    any of these companies. If the Mosley really is a rotating dummy
+AD4-    load then I say, let the truth be told. But before I go off and
+AD4-    trash someone's business, I should make sure that I have my facts
+AD4-    straight. The test results do raise an eyebrow towards Mosley, no
+AD4-    question about it. But I also see serious weaknesses in the test
+AD4-    protocol which could easily explain away the discrepancies. Without
+AD4-    further investigation, I would feel ill at ease towards lowering
+AD4-    the boom on Mosley. Anything else would be bad science.

I have an education in math. But there is a great deal in life that is
decided quickly and correctly without having to resort to sciobabble or the
second through seventh decimal place. According to Bell Laboratories, we can
barely detect double the sound level. The strength of our ears is dynamic
range, and parallel simultaneous discernment of tone, not discernment of
small changes in level.

We have tryouts for Little league. One kid throws from the outfield to home
on a line. The other throws home by lofting his throw somewhat higher. Guess
who plays center field and who plays second. It's true I didn't have the
kid's arm-twang measured to an absolute number of krbe's (equivalent twang
of a thousand rubber bands), and I didn't measure the height of the loft to
measure the implied correlated loss of kilo-rubber-band-equivalents. But if
you refuse to place the kids on your team based on those (non-scientific?)
insights, just because you CAN'T compute the krbe's, I'll have my team play
yours ANY day.

These are HAM RADIO antennas. Inexpensive antennas. Price out a 3 db TV
channel 5 transmitting antenna sometime. Ham radio antennas are a NICHE, low
margin market. Guess what+ACE-? There isn't the money in this business to do
standard FCC range tests on everything that moves. (JI isn't the only one on
this reflector who has had to cope with those. I have spent my share of time
in a helicopter with a field strength meter trying to prove or disprove
unwanted pattern anomalies at an actual transmitter site. Station owner paid
real MONEY for that.  No money-saving comparisons possible) Even at the
price they charge for the book, our daunting duo are essentially doing this
comparison for free. Just how many millions do we think they sold? Support
from the manufacturers? Oh, sure... Those guys have to pay the bills too.

If we blow off what these guys are starting to do, with a bunch of 3rd
decimal place criticism, or pan them for not doing the multi-kilobuck stuff
that the Broadcasters pay for, they'll just give up. They'll look at each
other and say we tried. And then we will be back to where we started. Is
that what we want?


+AD4-    +AFs-W4EF+AF0- I agree. I found the ad more incriminating than the test

I agree with your agree. In time, I thought of a few more words I could use
about the ad, but I won't.


Actually, maybe two years ago, I was at the point of buying a Mosley Pro-67.
Quite to my suprise, there was a fairly good group of contesters who told me
not to. There is still a part of me that likes that antenna. Mosley was the
real good stuff way back that I drooled over but couldn't afford. Someone in
PVRC gave me their old TH-3 to get me on in the SS and make a score for the
club. W3GRF gave me the tower and the RG17. I couldn't afford the TH-3 or
the tower or the coax or nuthin.  My wife was making all our children's
clothes at the time. Zero equivalent ham budget. Mosley +ACY- Collins were
impossible dreams. Those impressions don't go away easy. I personally wonder
if maybe Mosley has been milking that.

I went over to a ham's house around here and listened on 40 to a Pro-67 that
was up. The owner was getting ready to replace it. I also heard the
Cushcraft XM240 that replaced it. The owner was right. Number of traps?
Lossy traps? Traps vs. fewer number of loading coils? Don't really know why.
Just know that one chucks harder.

Scientific? Certainly not to some who have commented. But I have the right
kid in center and the right kid on second.

I have to repeat...  If this was a test of a single antenna, and the only
way to compare was to get accurate absolute measurements to compare with
other antennas done at different times and places, then I am 100+ACU- on your
side for the same list of reasons. This was a comparison of nearly
physically identical antennas all done on the same test jig. Some thing
do-able and affordable for a couple of essentially volunteers. Different
rules apply.

What does this comparison mean to me? I would rather buy straight aluminum.
Easier to put together and nothing fancy to go wrong. Traps, caps, and
gizmos can break, and the durn thing is up in the air.... It appears we
don't need traps, caps and gizmos anymore to get gain and three bands.

73 y'all, and may the one you bought play like gang-busters, and stay up
there, and stay working.   --- Guy.

(W4AN's location is gonna stomp me no matter what antenna I put in the air.
My only hope is a Georgia power failure...)

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