N4ZR wrote:
...."I can't speak for AO (MININEC based?), but my understanding is that
HFTA does not do an accurate job of reflecting the relationship between
boom
length and stacking distance."....
Both of these parameters require some consideration when modeling stacks
in HFTA. Since HFTA does not model mutual coupling between antennas,
close spacing (less than 1/2 wavelength) produces optimistic gain
results. There is a warning about this in manual.
The other item, boom length, is not a parameter considered by HFTA at
all. HFTA does not model the antenna. For an antenna of a certain
number of elements, HFTA assumes a gain that is typical for an antenna
with that many elements. As you know gain will vary with boom length
(as well as a few other items), but HFTA makes no allowance for that.
When comparing gain produced by two different antennas, you can make
some manual adjustments to compensate for this by increasing the number
of elements used for the antenna in HFTA to match the actual antenna
gain. Alternately if you know the actual antenna gain is, say maybe 2
dB better than that used by HFTA, then you know the plots will be about
2 dB low. This isn't normally a problem in HFTA because the main use
for HFTA is to see what the terrain does. However, if you are comparing
two different stacks of two different type antennas and want to know the
gain difference, then you have to make some manual adjustments.
Also watch out for the antenna that says they have X number of active
elements on a certain band, but in fact don't get any significant gain
from some of the elements. This often happens on multi-band antennas.
If you are stacking different antennas, results from HFTA could be
really shaky, as will be the results from any NEC based program, unless
you also model the matching network.
Jerry, K4SAV
Pete Smith wrote:
>I can't speak for AO (MININEC based?), but my understanding is that HFTA
>does not do an accurate job of reflecting the relationship between boom
>length and stacking distance. In the modeling I did with NEC-2 for my
>short-boom tribander stack, anything over 28 feet began to affect 15-meter
>performance (especially F/B) more than was acceptable. At the time, I also
>modeled TH-7 stacks with 24-foot booms, and at least in the terms I was
>concerned about, 30-34 foot spacing was about as much as I would have wanted.
>
>73, Pete N4ZR
>
> At 04:17 PM 3/20/2007, Joe Subich, W4TV wrote:
>
>
>
>>Steve,
>>
>>HFTA and AO show rather good results with 40/80 or 50/90 foot
>>stacking. The 40' spacing improves performance slightly on
>>20 meters and does not overly impact 10 Meter performance -
>>particularly if the 10 meter antenna uses most of the boom
>>length.
>>
>>73,
>>
>> ... Joe, W4TV
>>
>>
>>
>>
>>>-----Original Message-----
>>>From: towertalk-bounces@contesting.com
>>>[mailto:towertalk-bounces@contesting.com] On Behalf Of K7LXC@aol.com
>>>Sent: Tuesday, March 20, 2007 2:27 PM
>>>To: towertalk@contesting.com; k9kl@centurytel.net
>>>Subject: Re: [TowerTalk] Rohn 25 and stacking questions
>>>
>>>
>>>
>>>BTW have you modeled your proposed stacking? Forty feet
>>>seems like a long
>>>ways to separate them.
>>>
>>>Cheers,
>>>Steve K7LXC
>>>
>>>
>>>
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>>
>
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