[TowerTalk] 2 el yagi question

[W8AEF]

When I built the SVDA antennas for Clipperton/Midway/Kanton I used the 
following procedure:

Erect a parasitic element with a length close to a reflector for the desired 
frequency, just a little longer to allow for trimming (easier to trim than 
lengthen).

Erect the driven element of approximately the correct length, again a little 
longer to allow for trimming and isolated from the feed line.

Trim the driven element for resonance (0 ohms reactance) at the desired 
frequency.

Lower the parasitic element.

Erect a sample element about 100' away from the driven element and isolate 
it from it's feed line.

Starting 500 KHz below the target frequency I observed the AGC level at 100 
KHz intervals to 500 KHz above the target frequency for normalized signal 
levels.

Then I re-installed the reflector and repeated the frequency sweep to 
determine at what frequency maximum gain occurred.  When I had the frequency 
of maximum gain I then adjusted the length of the reflector to move the 
frequency of maximum gain to where I wanted it and repeated the sweep as a 
check.

Then I removed the reflector, shortened it until it was almost as shorter of 
the driven element as it used to be longer (as a reflector) and installed it 
as a director.

Repeated the frequency sweep to determine where maximum gain was, adjusted 
the length of the director element as required and repeat the sweep
as a check.

For the first array I then reversed the elements and repeated the sweeps to 
determine where maximum front to back was.

After the construction was fairly well complete, I started over with the 
sweeps to ensure that nothing has changed and I had a repeatable design.

Front to back was a curiousity, not a concern.  When you are on an island 
DXpedition you really want maximum forward gain and don't care about front 
to back.  I did find that front to back was not that great - to the point I 
was wondering where the gain was coming from.  I finally modeled the design 
in EZNEC and found the signal was squished down and that (to me) explained 
where the gain was coming from.

I found that the arrays could not be scaled from one band to another, each 
band array required a fresh start.  I concluded that was because of the 
proximity of the lower end of the elements to ground.  This distance was 
always about 4' above ground.  The idea was to get the tips as high as 
possible and still be able to reach the tip of the director so I could 
insert the strap that converted it to a reflector.

Because the measured gain was always higher with the reflector/driven 
element configuration that was the way the antennas were used.

In operation we found the antennas worked as well off the back as the front. 
Something about a salt water ground plane and being in a rare location, so 
we never did change from the reflector/driven element configuration.

The feed point impedance was measured at about 40 ohms.  After a hundred 
feet or so of coax was added the SWR at our desired operating frequency was 
1:1 at the amplifier end.

de Paul, W8AEF

ZF2JI/ZF2TA FO8DX/FO8PLA 8Q7AA ZX0A VU7RG TX5C A52PP

-----Original Message----- 
From: David Gilbert
Sent: Friday, May 18, 2012 10:26 AM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] 2 el yagi question


>From what I understand of the theory, a director is capable of slightly
(on the order of a tenth of a db) higher gain, but the tuning (spacing
and director length) is considerably more critical in order to achieve
that.  I'd have to check, but it might even be that case that in order
to get it optimized you'd need to adjust the driven element length as
well.  A reflector, on the other hand, gives it's best gain at wider
spacing and generally has a broader SWR curve, and also results in a
higher feedpont impedance for the driven element.  I have no doubt that
the combination of those effects could make a reflector give higher gain
in practice.

Unless space (or boom length on a yagi) is absolutely critical, I'd vote
for the reflector.

73,
Dave   AB7E