If you use the design in the ARRL Antenna book please be aware that the
taper schedule in incorrect?that is, the materials list does not provide a
snug fit between tapered sections and you will need to machine special
reducers to get a snug fit. I have a few spares available at no cost if
that happens.
On 10/1/13 3:45 PM, "Jim Lux" <jimlux@earthlink.net> wrote:
>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
>>
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