[TowerTalk] 80m vertical size calculation

Wed Sep 13 16:22:11 EDT 2017

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 at gmail.com 
Sent: Wednesday, September 13, 2017 3:39 PM
To: StellarCAT ; V. Sciucka 
Cc: Charles Morrison ; towertalk at 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 at 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 at 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 at 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 at 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 at 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 at contesting.com

>> > http://lists.contesting.com/mailman/listinfo/towertalk

>> >

>> _______________________________________________

>>

>>

>>

>> _______________________________________________

>> TowerTalk mailing list

>> TowerTalk at contesting.com

>> http://lists.contesting.com/mailman/listinfo/towertalk

>>

>