now this is where I get a bit frustrated ... MY experience in modeling
height change is that it doesn't reliably predict it! I am using NEC2
(although models checked have been re-checked courtesy of W8WWV using NEC4
and have been shown to be close) ... so possibly that is why - I've done it
with simple wire antennas just to be sure my model isn't complex (and the
proper segmentation based on AC6LA's recommendation and the gain check) ...
and now using Bob's real world results below they completely fly in the face
of Dan's results which show a 8X increase in comparison - 20 Khz actual
results vs. 70Khz showing an ~160Khz change (model) vs. his 20Khz!.
Now one could argue that the ground has a high degree of influence - but
that statement would completely invalidate using a model for real world
approximation for the vast majority of users since they have no clue what to
do re THEIR ground! It becomes at best a guessing game. So once again the
model only is a relative tool and although it seems to produce RELATIVE
results that are accurate its absolute results are quite suspect making the
tool much more an academic tool and not a real world one.
Anecdotal results are a very poor predictor of real results... after all
I've see (and I'm sure all of you have as well) a dipole at a decent height
perform extremely well...
Gary
K9RX
-----Original Message-----
From: Bob K6UJ
Sent: Monday, May 30, 2016 11:39 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Change in Frequency As Antenna Height Rises
Jim,
With my 2EL Force 12 40M yagi, I followed Tom Schiller's guidelines and
set it up initially at 15 feet
above the ground. When I raised it up to 70 feet the resonance
frequency increased by 20khz. From 70
to 90 about 2khz. This is what he said to expect in his manual. No
modeling or study done. Just
my results with the install.
I have tried the start out on the saw horse routine in the past with
yagis. The resonant freq ramps up on a
very steep curve and I had erratic results. On sawhorses, even the dog
walking by would change the tuning :-)
If the model shows that increasing the height from 30 INCHES to 60ft
results in a 215khz increase in resonant freq this could be realistic,
but I think we are spinning our wheels to do the initial tuning this
close to the ground.
73,
Bob
K6UJ
On 5/30/16 9:20 AM, Jim Thomson wrote:
Date: Mon, 30 May 2016 03:42:47 +0000 (UTC)
From: Dan Maguire<djm2150@yahoo.com>
To:<towertalk@contesting.com>
Subject: Re: [TowerTalk] Change in Frequency As Antenna Height Rises
Above Ground
K9RX (StellarCAT) wrote:
There's no "probably" to it! It is lowest (in frequency) closet to the
ground. [snip] Finally on 40 meters I saw as much as a 300Khz change
from 32" (on saw horses) to 37' on just one element of the yagi... I
believe it was about 150 Khz from 8' to 37'. And think there might be
an additional 20 Khz or so shift from here on up to probably 50' or so.
For a 40m dipole model (length 66 ft, diameter 1 inch, aluminum, 29
segments, over "Average" ground) the chart below shows (red trace) the
frequency of minimum SWR(50) and (blue trace) the frequency of resonance
(jX=0 at feedpoint) at heights from 2.5 ft (30") to 100 ft.
http://s33.postimg.org/vjtgi392n/resonate3.png
While it is true that the frequency (either one, take your pick) is
roughly 300 KHz higher at 37 ft vs 30", and is higher still up to about 60
ft, from there to about 100 ft the frequency *decreases* as the height is
increased. It also *decreases* from about 10 ft to about 25 ft. So it
seems that no blanket statement can be made. Whether the frequency
increases as the height increases depends on where you start looking at
things, at least for this simple model, which may or may not be an
accurate representation of Gary's one element from a Yagi.
Dan, AC6LA
## OK, so resonant freq increases from 7025 to 7240.... when height
changed from 30 ft...to 60 ft. Great, so how the hell am I to tweak
each ele on my 3 el 40m yagi ??
F12 sez to tweak the REF at 6910 DE at 7070 DIR at 7500. (
340N) I find it really hard to believe that the resonant freq will
increase by 215 khz.... simply by changing height from
30 ft to 60 ft. Thats a huge amount. Something is amiss in the
model.
Jim VE7RF
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