At 02:15 AM 2/1/96 -0500, Walt Kornienko wrote:
>Tom, W8JI writes:
>
>> Put in a good ground system and shunt feed the tower. I like to use a cage to
>> minimize the Q and improve the bandwidth, and reduce the voltage across the
>> series cap. I never use an omega match, instead I adjust the tap point
>> between the shunt wires and tower for lowest SWR with only a series cap. That
>> gives maximum BW and efficiency, because the series L/C circuit formed by the
>> cap and the gamma wire has the lowest L/C ratio and the lowest loaded Q of
>> any shunt feed system.
>>
>
>Tom, I always enjoy your very helpful technical analysis and advice.
>I'd like to pose a few questions and make some comments on
>observations I've made while modelling different feed systems
>for my yagi-on-top loaded tower.
>
>According to a graph in ON4UN's book, a tower about 30m high,
>with a face of about 12", and topped off by a common 4 element
>20m monobander on a 36' boom has an electrical length of about
>100 degrees. I have wondered how he derived the graphs,
>if there is a simple function that can be calculated based
>the variables in the given problem.
>
>For shunt feeding a tower he directs the reader to another graph
>where he shows the height of tap point -vs- electrical length,
>for three gamma wire spacings, .5m, 1.0m & 1.5m. He also gives
>instructions on how to emperically derive the electrical length
>by coupling a GDO to a wire that runs from the top of the tower
>to ground level.
>
>All well and good if you do not stack antennas. Many stack one
>or more yagis on the same tower, and many are limited to using
>that tower for 160M. Thats pretty much the case here.
>
>The stacking situation is further exacerbated by the fact that many
>can not place the gamma tap point below the lowest beam in the stack.
>I found that if I used a wide spacing, about 5' from gamma wire
>to tower, I could achieve a reasonable match to 50 ohms. In my case
>42' seemed to work OK. The lowest beam on my tower is planned to
>be at 45'. So far no problem... until I modelled a 34' boom 3' above
>the gamma tap point. This spelled disaster! The model showed extensive
>coupling between the gamma feed and the boom. The effects on the 160m
>antenna were a distorted radiation pattern, a drastic change in
>drive impedance & reduced gain.
>
>Are there any ways to minimize these interactions?
>
>Would the N4KG reverse feed method be a better choice for such a
>situation.
>
>*****************************************************************
>* *
>* 73 de Walt Kornienko - K2WK > W3MM (FRC) *
>* K2WK@crystal.palace.net or K2WK@N2ERH.NJ.NOAM *
>* *
>*****************************************************************
Walt, I ran into the same problem after modelling, so I never implemented
it. I put up an inverted "V" at the top of the tower (134 ft/40 M) and it
plays fantasticlly!!
Bill, N3RR
73!
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