Together with Jim, SM2EKM I have recently done some testing of f/b on my
stack of TH7DX's.
The f/b on the single TH7DX tribander over a wide frequency range is
essentially the same. The difference over 300-500 khz is only 2 db at the
most. We tested at every 100 khz going from the bottom band edge to the top
band edge, except on 10 mtrs where we went from 28.000-28.600.
The distance between the two stations is about 30 km and the ground is
almost flat. We have ground wave all the time.
Front/back on the single top yagi was best on 20 m, abt 17-18 db but
improved about 3db when I switched to the stack. On 15 and 10 mtrs the f/b
is about 15 db and there is not much difference when I switch to the stack.
Of course, when operating normally and sky wave is involved f/b differs
quite a lot depending on the incoming angle of the station I am hearing.
Sometimes I see >25db and other times less than the nominal 15-18 db.
73/Peter SM2CEW
At 11:56 2004-06-23 , Hasan wrote:
>
>The only point I would contest, based on my own experience and measurements,
>is the lumping in of F/B as an advantage for the typical tribander. I didn't
>find this to be the case, even comparing it to a 203ba....3 el 20m
>monobander. I found it to be 25 dB in a selected spot, but moving 150 kHz
>away, saw the F/B deteriorate markedly.
>
>Even well designed tri-banders do not do a good job of maintaining F/B
>across the 20m band. They can have a very high F/B over a VERY narrow range
>of frequency, but they do not maintain this front to back going, for
>example, from 14.025 to 14.300. These same antennas do a much better job of
>maintaining forward gain across the 20m band, but in F/B, they did not fare
>well. To be fair, a monobander can be designed for better front to back
>across a wider range than the typical settings provide, but there is always
>a sacrifice in gain when this kind of optimization is done.
>
>With respect to the Tennadyne T-10, I made some specific measurements using
>a step attenuator and found that the F/B was no worse than 17 dB at every
>frequency I checked (sampled approximately ten points per band for all ham
>bands from 13 to 30 mhz) It was greater than 22 dB at many of these points,
>and greater than 25 dB in a few selected spots. The tribanders I compared
>this to were nowhere nearly as good, once one moved away from the frequency
>of best f/b. This same pattern of consistent F/B was maintained outside the
>ham bands as well.
>
>In general, the forward gain of the well designed tribander will be better
>throughout the common frequency range of a similarly sized LP. The match
>will probably not be nearly as good as the LP, and from my experience and
>measurements, the F/B of the LP will stay consistently higher throughout the
>common range of the two antennas, EXCEPT at one narrow band of frequencies
>where the tri-bander or monobander will be considerably higher than the LP.
>
>The advantages compared to tribanders of the Tennadyne LP's I have used:
>
>1. Relatively lower cost.
>2. Lower or equal weight to a tribander.
>3. Lower wind loading.
>4. Mechanically more sound.
>5. No traps to fail, virtually no maintainence.
>6. Better consistency in F/B
>7. Better match to a wider range of frequencies.
>
>Disadvantages:
>
>1. Lower forward gain by at least 2 dB
>
>Summary Comments:
>
>The Tennadyne LP's are light, ruggedly designed and excellent performers.
>They are not killer contester antennas, because they lack the ultimate
>forward gain of a monobander or very well designed tri-banders.
>
>The one thing I can say as far as "on air performance" is that I get
>virtually everyone I call, in the first or second attempt, even in large
>pileups. I do call when the band favors me. Kenwood TS-870s > ALS-600
>TMOSFET PA (600w) > Tennadyne T-6 @ 52', rural area, excellent HAAT, low
>noise.
>
>Hopefully these observations will help someone in the market for a good, but
>not perfect antenna that is a fine performer, mechanically sound and
>reasonably priced.
>
>...hasan, N0AN
>
>
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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