Hi Kim,
While 50 dB F/B is certainly possible with a 3 element
yagi, it would generally only hold up over a very narrow
frequency range and a very narrow range of elevation
angles (imagine a deep, but narrow null off the backside
of the antenna). Under real life propagation conditions,
the angle of arrival of a signal propagated over an ionspheric
path is not constant, so it would be unlikely that the signal
in question would "stay in the deep null" for any great
length of time. Given the dearth of accurate S-meters
found in commercial amateur equipment, I would put my
money on an inaccurate S-meter (my Ten-Tec rig
is horrible).
If you can, get yourself an accurate step attenuator and
then find yourself an S-9+10dB signal (preferably a local
ground wave signal that will be devoid of fading). Adjust
the attenuator until the S-9+10dB signal is S1 and read
off the attenuator setting in "dB". This will give you a much
better indication of your antennas F/B in the example you
cited.
You can also measure F/B back in real time using the
step attenuator. With minimum attenuation just turn the
antenna away from the desired station and note the
S-meter reading. Then turn the antenna 180 degrees
so that you are pointing towards the desired station
and then adjust the attenuator until you get the same
S-meter reading as in the reverse direction. This is
the F/B ratio for that particular angle of arrival and
operating frequency. This sort of measurement will
be subject to errors due to fading, so its best to
do a lot of measurements on different stations at
varying distances from your QTH. After you do
a number of measurements, a pattern will emerge
that will give you an indication of the antennas
typical F/B performance. Once you establish this
baseline, you can fool with element lengths from
the shack (very cool!) to see how they effect
F/B performance.
BTW, if you don't have a step attenuator, Pete,
N9SFX makes a nice step attenuator kit that is
perfect for this sort of application. He supplies a
machined aluminum housing with cavities for each
attenuator cell, all the resistors, and the toggle
switches. The kits I bought were $57 each.
You can reach Pete at N9SFX@aol.com
Have fun - sounds like a neat antenna.
73 de Mike, W4EF.........................
----- Original Message -----
From: "Kim Bottles" <kbottles@rafn.com>
To: "TowerTalk" <towertalk@contesting.com>
Sent: Wednesday, December 12, 2001 3:05 PM
Subject: [TowerTalk] Front to Back Measurements
>
> Greetings Ladies & Gentlemen,
>
> I claim to know very little about antenna theory. However I have noticed
> something on a practical level with a new antenna and wonder if any of
> you might be willing to comment on it and clarify for me what I have
> observed.
>
> I recently purchased a SteppIR variably tuned Yagi. (See page 129 of the
> December QST or visit their website at www.steppir.com)
>
> This is a three element yagi with the ability to tune each element
> remotely from the shack. (Look at their website, they explain how it is
> done better than I could here.)
>
> I was listening to a fairly long winded SSB QSO last night on 20 meters
> and had time to make several measurements of front to back on a loud
> station. The SteppIR has a one button swapping of director and
> reflector, so flipping the direction 180 degrees takes but a few
> seconds.
>
> The loud station was in North Carolina and I live on Bainbridge Island,
> just west of Seattle, a distance of about 3000 miles. With the yagi
> pointed at the Carolina station he was registering a solid 5 dB over S-9
> with peaks to 10 dB over S-9. Flipping the director and reflector
> dropped him down to barely peaking S-1. Is this really 50+ dB front to
> back? The rig used was an Icom 756Pro. My yagi is at 60 feet in the
> clear.
>
> I have noticed the same kind of results on other stations, but this was
> the first situation where I was able to switch back and forth numerous
> times with the same results. (Thank goodness for long-winded SSB QSO's.)
>
> I know this is not a valid scientific experiment, so please do not
> attack me as if I was claiming it was. I am only reporting an antidotal
> (as K7LXC would say) observation. (Is that how you spell "antidotal?)
>
> Because this antenna is brand new to the market (and a heck of a lot of
> fun to play with I might add), I thought some of you might find my
> observations interesting and feel moved to comment.
>
> Comments on this reflector always seem entertaining and sometimes
> educational.
>
> Best Wishes for a Happy Holiday Season,
>
> Kim Bottles - K7IM
>
>
>
> List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
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>
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>
>
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!! http://www.anwireless.com
-----
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