Your 10 meter stack at a 0.57 wavelength spacing is not a close spaced
stack. The 20 meter stack you are considering at 0.36 wavelength spacing
is a close spaced stack. The answers you get from YO should be correct.
YO should show less gain from this stack than HFTA does. HFTA starts
being significantly different somewhere around 0.5 wavelength spacing.
HFTA will always show 3 dB gain regardless of the stack spacing. This
is not the way it works. HFTA uses a point source for the antenna, and
ray tracing is done in only the forward direction. Rays directed toward
the other antenna are not considered.
I just didn't want you to be mislead by the gain numbers produced by
HFTA which are in error for close spacings. You should be able to trust
the gain numbers from YO.
Another thing to consider if you are trying to use exact gain numbers
from HFTA is that HFTA uses fixed gain values for each antenna. It does
not do field analysis like YO or NEC. For example, a four element yagi
always has 8.5 dB free space gain. This was chosen as a typical number.
In the real world your 4 element yagi may have a different gain value.
If you are looking for precise gain numbers for comparison to different
antennas or to the output of another program, you have to do a manual
gain correction. If you are using HFTA just to look at the effects of
terrain, then a gain error of a dB or two doesn't matter.
Jerry, K4SAV
Rick Dougherty wrote:
>see AA4LR's reply...low stacks will still perform excellently as the average
>terrain is increased through the sloping effect...I can still see decent
>gain figures with 0.5 wavelength spacing with both HFTA and YO...even with
>fairly long boom antennas and I have used such antennas with a great deal of
>success...again my 10m JA stack was a laser beam to JA...it consistently
>beat my other single antennas..and we always got comments like first East
>Coast or last East Coast in JA....de Rick nq4i
>----- Original Message -----
>From: "K4SAV" <RadioIR at charter.net>
>To: <secc at contesting.com>
>Sent: Monday, June 12, 2006 10:16 PM
>Subject: Re: [SECC] Simple Antenna for 6 Meters that Works
>
>
>
>
>>..."I say again a stack will beat any single antenna 90% of the
>>time...period...de Rick nq4i"...
>>
>>Over flat ground I will agree with you, and for MOST other locations I
>>would also agree, but there are locations with sloping ground where it
>>is easy to get an antenna too high. It is these locations where a stack
>>may show little or no benefit over a single antenna.
>>
>>Also remember that finding a location with sloping ground in all
>>directions is hard to find. If part of the azimuth goes uphill, then
>>you are going to be worse off than a flatlander in those directions.
>>
>>If you are using HFTA to model a 20 meter stack with spacing of 65 and
>>90 ft, you need to correct the resulting gain HFTA gives you. This was
>>the reason for my caution about HFTA with close spaced stacks. It
>>considers the antennas to be point sources. You can actually put two
>>antennas at the same height in HFTA and it will show 3 dB gain. If you
>>could do this in the real world, the gain would be zero. Here are couple
>>of references on stacking distances.
>>http://www.w8ji.com/stacking_broadside_collinear.htm
>>http://www.ifwtech.co.uk/g3sek/stacking/stacking2.htm
>>
>>Jerry, K4SAV
>>
>>Rick Dougherty wrote:
>>
>>
>>
>>>I must weigh in on this topic...I have had considerable success with
>>>
>>>
>close
>
>
>>>spaced stacks...we had a 10m stack at 55 and 75 ft to JA and it was
>>>awesome...right now I am giving seroius consideration to a 4 over 4 stack
>>>for 20m at 65 and 90 ft...HFTA shows that this is a very awesome
>>>combination....I dare anyone to work more qso's in more areas with out a
>>>stack...the footprint on the other end is what it is all about...I say
>>>
>>>
>again
>
>
>>>a stack will beat any single antenna 90% of the time...period...de Rick
>>>
>>>
>nq4i
>
>
>>>----- Original Message -----
>>>From: "Tommy" <aldermant at alltel.net>
>>>To: <secc at contesting.com>
>>>Sent: Monday, June 12, 2006 9:10 PM
>>>Subject: Re: [SECC] Simple Antenna for 6 Meters that Works
>>>
>>>
>>>
>>>
>>>
>>>
>>>>>The conclusion is that there is no stack at my location that will
>>>>>beat a single antenna. Your mileage will probably vary. This is a
>>>>>
>>>>>
>unique
>
>
>>>>>solution for my location, but it could be duplicated at other selected
>>>>>spots.
>>>>>
>>>>>A side note: HFTA does not model close spaced antenna interaction, so
>>>>>stacks placed closer than about 0.5 wavelength will produce optomistic
>>>>>results.
>>>>>
>>>>>Jerry, K4SAV
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>Jerry,
>>>>
>>>>Quite interesting!! So I assume your located in the mountains? Did
>>>>you find your best solution to just stick with a single yagi? What
>>>>
>>>>
>height?
>
>
>>>>Also wonder what model program you use to model your various antennas
>>>>within, say' a 1000' circle?
>>>>
>>>>What ever antenna I have will most likely be down by this time
>>>>tomorrow as that 'tropical storm' is coming smack through Valdosta
>>>>tonight and tomorrow!
>>>>
>>>>Tom - W4BQF
>>>>
>>>>
>>>>
>>>>_______________________________________________
>>>>SECC mailing list
>>>>SECC at contesting.com
>>>>http://lists.contesting.com/mailman/listinfo/secc
>>>>
>>>>
>>>>
>>>>
>>>>
>>>_______________________________________________
>>>SECC mailing list
>>>SECC at contesting.com
>>>http://lists.contesting.com/mailman/listinfo/secc
>>>
>>>
>>>
>>>
>>>
>>_______________________________________________
>>SECC mailing list
>>SECC at contesting.com
>>http://lists.contesting.com/mailman/listinfo/secc
>>
>>
>>
>
>
>
>
|