One should be able to get within say 3% .... 5% tops to start with – right?!
Measuring the actual frequency of the actual antenna and changing it this much
will work ... if done properly – in one go. I’ve done it many times with
tapered elements. Again – you’re not changing or impacting the antenna enough
for a simple ratio correction to be off by much. I’m not saying that if you
measure it to be 3468 Khz and you want it to be 3584 Khz that it WILL be at
3584 ... but it will be close and in the right direction. For most purposes
good enough. That is all that one needs – right? We’re not looking for
perfection here. Or at least I am not/was not.
I totally agree with you Chuck that IF the antenna were widely tapered – if you
change it substantially either in length or diameter it is going to be off –
and if that is indeed the case then fine – “maybe” he does it twice.
As for modeling – yes – you can obviously do that and should do that. But my
experience has been that modeling generally gets you in the ball park – you
might still need a small tweak to that. So I’d start with a model ... but
testing the ACTUAL antenna in situ can’t be beat. It is THE info you needed –
precise for that antenna in that location.
What I would do and have done is I would model the antenna as it is to be built
or is built ... test it ... but here’s where people start to get confused re
using a model, if they’re not totally familiar with changes like this. So you
model for a resonance of 3600Khz. You build it precisely to the model and test
it – its actually at 3540Khz. If you have to have the original design frequency
now what? Well you go back to the model – you change the tip length to effect
the DIFFERENCE in frequencies – i.e. you need it to be 60Khz higher – so you go
back to the model, shorten it to get a new frequency of 3660Khz! This change in
length is what you would do to the antenna. The same DIFFERENCE that you
need/saw in the actual antenna. Again: this is not precise – but it will work
just fine.
g.
looked at another way: (didn’t know this was going to be controversial :>) )
-tapering effectively gives you an average diameter.
-so his antenna starts at 4” and tapers to 1”. Lets say the average is 2.5”.
That is more than likely not what the model would come up with but its probably
close ...
-a 67’ vertical that is 2.5” in diameter has a resonance of 3.517Mhz roughly
give or take a couple Khz.
-if I change the effective diameter by 10% – MUCH more than I believe we’re
talking change here which is my whole premise, to 2.75” the resonant frequency
in the model changes – whoops... about 3 Khz!
-even changing it to 3”, a pretty substantial effective diameter change – moves
it only ~6Khz.
ok?
From: w5prchuck@gmail.com
Sent: Wednesday, September 13, 2017 3:39 PM
To: StellarCAT ; V. Sciucka
Cc: Charles Morrison ; towertalk@contesting.com
Subject: RE: [TowerTalk] 80m vertical size calculation
I agree. But, what was wanted is a formula where only one adjustment was
needed. I think that will only work when the entire element is one diameter
such as a wire. The ratio will get close, but will probably need at least one
more “tweak.”
Chuck W5PR
Sent from Mail for Windows 10
From: StellarCAT
Sent: Wednesday, September 13, 2017 2:24 PM
To: V. Sciucka; Chuck Dietz
Cc: Charles Morrison; towertalk@contesting.com
Subject: Re: [TowerTalk] 80m vertical size calculation
these are only starting points! Local conditions - the antenna itself, the
surroundings, the ground, etc etc will all conspire to move that value up or
down slightly. KNOWING what your particular antenna is in place - that is
the perfect starting point.
g.
-----Original Message-----
From: V. Sciucka
Sent: Wednesday, September 13, 2017 2:46 PM
To: Chuck Dietz
Cc: Charles Morrison ; towertalk@contesting.com
Subject: Re: [TowerTalk] 80m vertical size calculation
I just thought that it might explain why different formulas are used.
Charlie gave me formula which is widely available: 246/f(mhz) = element
(feet), but I also found https://www.dxengineering.com/
techarticles/verticalantennainfo/dx-engineering-comtek-verti
cals-for-phased-arrays (see p.5) where 234/f(mhz) is used.
--------------------
Vytenis
2017-09-13 21:33 GMT+03:00 Chuck Dietz <w5prchuck@gmail.com>:
> Maybe I don't understand, but I would think that if you had a tower with
> an aluminum tube "stinger" on top, this might not work because of the
> change in percentage of the various diameters.
>
> Chuck W5PR
>
> On Wed, Sep 13, 2017 at 1:00 PM Charles Morrison <junkcmp@gmail.com>
> wrote:
>
>> Not length to diameter, It is independent of diameter.
>>
>> It is a simple method to determine a difference of length as a ratio
>> based
>> on frequency.
>>
>>
>> On Wed, Sep 13, 2017 at 11:41 AM, V. Sciucka <vytenis.sciucka@gmail.com>
>> wrote:
>>
>> > Thanks Gary K9RX and Charlie N1RR, 2nd question is clear now.
>> > Charlie also gave formula for length 246/f(mhz) = element (feet) which
>> I
>> > assume includes length to diameter ratio or this ratio is not so much
>> > important.
>> >
>> > --------------------
>> > Vytenis
>> > _______________________________________________
>> >
>> >
>> >
>> > _______________________________________________
>> > TowerTalk mailing list
>> > TowerTalk@contesting.com
>> > http://lists.contesting.com/mailman/listinfo/towertalk
>> >
>> _______________________________________________
>>
>>
>>
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>
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