[TowerTalk] 40 meter rotating dipole question
w2lu at rochester.rr.com
Tue Oct 1 20:50:25 EDT 2013
Hi Mark -
It sounds as though you have a good project underway. Just remember that,
generally speaking, with antennas, as with so much of life "there's no free
lunch". Yes, you may be able to satisfy your appetite but there will be some
sacrifice. The shortened dipole may work quite well on the frequency it is
tuned for but there will be some sacrifice in gain and more particularly in
bandwidth. As for gain, as a first cut, imagine the area under 180 degrees
of a sign wave. Imagine it as the current, which does most of the radiating,
in the element. If you cut some off each end you don't loose too much of the
area or current/power, or in our case the element "gain". So the further
out the element you place the correction factor the less you loose in
"gain". That's why capacitive end loading is attractive electrically, if not
mechanically.You are however still left with a relatively low radiation
resistance at the feed point which will require a matching device. With no
element loading you're left with both a low radiation resistance and a
reactance which must be delta with. Each case requires a matching device
which will add power loss.
So, my take on it is that yes, you can shorten the element, add some
reactive loading, and an efficient feed point matching circuit, tune it all
for the desired operating frequency, and come up with a relatively good
dipole with a relatively limited bandwidth.
Gene / W2LU
----- Original Message -----
From: "Mark, K5ER" <k5er at arrl.net>
To: <towertalk at contesting.com>
Sent: Tuesday, October 01, 2013 3:24 PM
Subject: [TowerTalk] 40 meter rotating dipole question
> 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?
> Question two - IF a shortened antenna is better, is there a "better"
> length? I see many in the range from 38 to 55 feet.
> Question three - If shortened, I know the ant will be capacitive, and
> therefore can be matched by adding an inductor across the feed. 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 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.
> Mark, K5ER
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