[TowerTalk] 40 meter rotating dipole question

[Jim Lux]

On 10/1/13 12:24 PM, Mark, K5ER wrote:
> Hi Guys, I have checked the archives, but not found specific answers, so
> forgive me if this has already been covered and I am asking the wrong
> questions. To help a friend, I am building a 40 meter rotatable dipole,
> to live at 90' on his mast. It will be about 12' above his 36' boom 20M
> OWA. I will fabricate a heavy duty mount/center insulator, and
> incorporate 2 truss cables, either dacron or kevlar.
> He delivered to me two (used) elements from a previous unidentified
> antenna. Each element is abt 28' long, with the taper schedule going
> from 1 1/4 down to 5/8. I have spare material in stock, so I could
> easily add a 1/2" x 5' tip to each and end up with a full size dipole.
>
> Question one - The price of material isn't much more, so why does it
> seem that "nobody" builds rotatable dipoles to the full 66-67 feet?

Practicality?  Longer elements require stronger elements, weigh more, etc.



>
> Question two - IF a shortened antenna is better, is there a "better"
> length? I see many in the range from 38 to 55 feet.

There's very little performance difference between a full length dipole 
and one that is, say, 1/2 length (e.g. 1/4 wavelength overall).  The 
difference is that the SWR bandwidth will be narrower, and there's some 
potential loss from however you managed to make the short dipole provide 
a decent match.  I say potential, because the inductive loading it takes 
just isn't that much, and it's easy to make a coil with a Q of 100. 
Compared with the antenna Q of <10, that's not a big deal.

I suspect the shorter lengths are based on mechanical convenience more 
than some rigorous analysis of electrical performance.

>
> Question three - If shortened, I know the ant will be capacitive, and
> therefore can be matched by adding an inductor across the feed.

or in series.
Or a tapped coil

  I find
> many pages of formula with lots of Latin characters, but missed that day
> in math class. If someone had a hint of a starting point, ie> 4" long, 5
> turns on 2" diam. , or 4 turns on 3" diam, etc. I could then work
> backwards and make this work for him.

I'd take a look at the numerous designs posted for a "shorty forty" and 
work from there.

The problem you face is that as the antenna gets shorter, the resistive 
part of the impedance gets smaller.  So your tuning network (be it LC or 
L transformer or something else) has to do two things: add inductance to 
cancel the capacitive reactance and transform the low R up closer to 50 
ohms.

There's lots and lots of ways to do this: Various and sundry feed 
networks (gamma match, double T match, etc.) provide both the 
transformer and reactance in various ways.  Mostly it's a matter of 
mechanical convenience.  High gain Yagi-Uda antennas tend to have low 
feedpoint impedances, so feed schemes that work for a Yagi are likely to 
work for your shortened dipole, perhaps with the addition of some 
inductance.

A given manufacturer will tend to choose a method *different* from other 
manufacturers, so as to provide a differentiating aspect for 
advertising.  Use our fabulous low loss autotransformer with silver 
plated coils instead of Brand Y's parallel transmission line loading or 
Brand Z's continuously loaded spiral or Brand X's lumped LC network.

As long as you're not trying to make an antenna that is a tenth of a 
wavelength long, there's not a huge difference.

>
> I have EZnec 5+, but have only used it to build OWA antenna so far,
> which makes a beautiful direct feed, so I have no experience with adding
> a load. I plan to input the taper schedule into EZnec, but if a given
> length is known to be superior, I am not too proud to ask for it, rather
> than spend hours building model after model working my way through the
> possibilities.
>
> I understand that height can affect performance and feed point
> impedance, however, neither the owner nor I climb. I need to model and
> build this antenna, and then expect it to work as modeled when the
> climber puts it in place, so any ideas would be appreciated.
>
> Thanks,
> Mark, K5ER
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>