[TowerTalk] STACKED 40 METER BEAMS MATCH
Joe Subich, W4TV
lists at subich.com
Thu Nov 25 12:50:06 PST 2010
> To get the best comparison it would be helpful if you had the
> physical characteristics for the Yagi's that Dean uses as the basis
> for his antenna selections in HFTA.
Dean has made it clear that he uses a simple isotropic source of the
specified gain. Realize that the use of an isotropic source (e.g. no
F/S in the vertical plane) also effects the final pattern by including
excess contribution from RF striking ground close to the tower that
may not exist is a practical antenna (particularly longer boom yagis).
... Joe, W4TV
On 11/25/2010 1:33 PM, Michael Tope wrote:
> On 11/25/2010 2:30 AM, Jim Thomson wrote:
>> ## The ARRL HFTA program is flawed somewhere. Apparently it uses simple
>> ray analysis in the far field only. In the old w2pv literature /notes, w2pv modeled
>> a 20m yagi at 150'.... and varied the height of the lower yagi from 65/75/85'. The BIP
>> gain peaks when using the lower yagi at 75'..and drops off at 65/85. HFTA on the
>> other hand shows the gain increasing, as the lower yagi keeps getting higher and higher.
>> Try it, it's bizarre. Gain is lowest, when lower yagi is say 65'...then just keeps on increasing, as
>> you raise it!
>> When the bottom yagi is a 130', it's still increasing. [top one stays put at 150']. Since the HFTA
>> program does not factor in near field interaction/mutual coupling, etc, that might explain the dismal
>> results. [ it was w2pv who 1st suggested the BOP mode]. Who knows, maybe at the top of the
>> next cycle, [ when angles are usually a little higher], BOP may actually work...more often.
> I believe you are correct about HFTA, Jim. From the HFTA manual:
> "The internal Yagi model in HFTA is a simple mathematical model.
> It does not compute interactions between individual Yagis in a
> assumes that each antenna is a "point source." For antennas stacked
> more than about a half wavelength apart this is not a problem. For
> you should be cautious specifying spacings less than about 20 feet
> on 20
> meters (and proportionately scaled on other bands) because of
> mutual-coupling effects between real antennas.
> An interesting experiment would be to compare the output of EZNEC with
> HFTA (terrain set to flat ground) using the 20 meter stacked Yagi
> parameters you give in your example. To get the best comparison it would
> be helpful if you had the physical characteristics for the Yagi's that
> Dean uses as the basis for his antenna selections in HFTA. I would
> expect to see good agreement between the two programs at wider Yagi
> spacings with the results diverging significantly as the spacing was
> 73, Mike W4EF...................
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