Roger -
I've been on 6M since the mid-90's, with various antennas an power levels.
An important thing to remember about 6M is that a wavelength is only
about 20', so as you raise its height, an antenna quickly acts as if
it's in free space, and the takeoff angle is barely affected by the ground.
One lesson I learned the hard way is that "higher is most definitely
NOT" always better on 6M. For years, my high antenna has been a 7/7
stack at 160'/140', and for as long as I've had it, I've felt that I
don't hear very well. During the last cycle peak, I was regularly
outgunned by a local who was using a much lower and worse antenna, and
he was able to work a lot of new ones that I couldn't even hear. I've
re-added a low antenna 6L at 40' to the mix, and find that it's much
quieter, and I'm frequently able to hear with that what I can't with the
7/7.
This all made sense when I finally realized that the high antenna, while
incredible for ground-wave QSO's, the reciprocal was that also hearing
noise from that many more local sources. Some quick math shows that
the high antenna is exposed to ~4.5 times more local noise than the 40'
antenna to local noise. I frequently observe a 8 - 10 dB higher noise
floor on the high antenna.
Toss in loss due to the coax having to be longer to reach the high
antenna, and the case for a high antenna gets even worse.
A couple of weeks ago, I worked 4L/DL7ZM via terrestrial path on 6M - a
6,500 mile distance - on the low antenna, because it heard him better
than the high antenna (which is currently only 7L due to a problem with
the phasing harness).
Regarding Sporadic E prop:
- QST, August 2016: "The World Above 50 MHz" (which includes a link to:
http://k9la.us/html/vhf.html)
-
http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1993JIMO...21..182M&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES
Hope this helps,
73,
Steve
NN4X
> Date: Sun, 31 Jul 2016 01:50:04 -0400
> From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>
> To: "towertalk@contesting.com" <towertalk@contesting.com>
> Subject: [TowerTalk] 6-meter antenna height
> Message-ID: <7755882c-91f3-ec85-c951-c857103c252b@tm.net>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> FD operation got me to thinking about this.
>
> On HF (depending on band conditions) there is a statistically preferred
> height for the distance to be worked.
>
> On VHF particularly 6-meters where long haul is Sporadic E, Tropo,
> occasionally F, or Aurora.
>
> Local work is usually the higher the better. I've normally had the 7L
> C3i (boom just a few inches short of 30 feet) at 115 feet and excellent
> results for single and double hop Sporadic E as well a F and Au.
>
> Keeping in mind that the ideal height for HF is statistically derived,
> where is the point (frequency) where we should typically be switching
> over from the way we figure HF antenna height to the higher the better?
> The FD station had their 6-meter antenna height calculated the same way
> we usually figure HF antenna height, meaning, I could about jump high
> enough to touch the antenna. That is, if I could still jump. They were
> shooting through the trees.
>
> A typical 6-meter antenna would have a "bunch" of lobes between the
> horizon and vertical
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|