Bill: I think you are wasting your time with the trap dipoles. You can use
the parallel dipole designs which are much more efficient as well as
simpler. The same principle can be used for paralleled verticals. If you
want to use coax feed line for the dipoles, you need an effective balun at
the coax attachment point. I suggest you look carefully at the newest balun
designs which you can either build yourself or buy from DX engineering. K9YC
has some excellent designs for really effective baluns using ferrite cores
operated ;in the resistive mode to develop wide band isolation of 5000 ohms
or more. The type 31 ferrite they use is available from Mouser electronics
and others. The baluns we have been using for years are generally lacking in
high common mode rejection (keeps your coax shield from radiating).
If you want to build some directive antennas look into the Moxon 2 element
yagis. They have wide band F/R and a feed point Z of around 50 ohms. There
is a very interesting web site devoted to the Moxon antennas. I don't have
the URL handy but am sure you can find it with a Google search. Lots of
simple wire Moxon designs there from all over the world. Good luck with your
antenna designs.
73, Dan, N5AR
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com]On Behalf Of William Moore
Sent: Wednesday, June 18, 2008 1:13 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Trapped in antenna trap confusion
Hello,
I seek help in resolving some confusion with respect to antenna traps.
I am an Old Timer, who has been passively active in Amateur Radio since
first licensed in my early teen years. Now retired, I want to pick up from
my all too short youth and recapture the awe, glory, excitement and
satisfaction of radio magic that comes from kit builds and home brew station
set-ups. Winter is now over, my K2 is complete, the J-38 key cleaned, and
my code speed is approaching a respectable 15 wpm. So, with spring in the
air, an OT's fancy turns to ............well, ummm, thoughts of
constructing some simple home brew antennas.
My bands of interest (until higher band conditions improve) are 75, 40 and
20 metres. Open wire feeders to my antenna garden are not at all possible.
Access to the antennas will be 100 foot conduit runs of RG-8 coax to the
antenna locations. I also live on a rock pile which is well covered in a
dense forest making ground mounted vertical installations impractical. Trees
for dipole supports are limited to about 30 feet likely resulting in NVIS
like dipoles for 40 and 80. What I have in mind from my literature review
are to construct a horizontal 80/40 trap dipole and an elevated 40/20 trap
vertical. The dipole design would be a classic W3DZZ type or perhaps the
newer style W8NX type using coaxial traps. These two dipole antennas are
described in the 19th Edition ARRL Antenna Book or at
http://www.arrl.org/tis/info/Trapped.html
<http://www.arrl.org/tis/info/Trapped.html.> . My proposed vertical is
based on an article "Four Bands on a Pole" by W4DWK/W1CQS which appeared in
the September 1972, QST, or at
http://www.arrl.org/tis/info/Vertical-H.html. This vertical antenna is
essentially a 1/4 wave 20 metre element terminated into a 14.150 Mhz trap,
which is followed by a 10.5 foot element extension beyond the trap to
provide a reasonable match at 40 metres.
However, questions and confusion conflict my decisions
1. Should I forget using coaxial traps in favour of the W3DZZ classic coil
and capacitor traps? W8JI offers some compelling arguments against coaxial
type traps in his speaking notes on this subject located at
http://www.w8ji.com/traps.htm. I draw the conclusion that coaxial traps are
too low in Q, lossy and inefficient for an effective trap. However, are they
sufficiently poor that they should be removed from consideration?.
2. What should be the design frequency trap of antenna traps? W8JI
recommends in his notes that the trap resonant frequency should NOT be at
mid-band. For instance, he suggests 6.5 Mhz for a 40 metre trap. On the
other hand, the ARRL Antenna book recommends that a 40 metre trap should be
designed for 7.150 Mhz. Also, how would one ever calculate the correct
frequency for other bands using W8JI's trap frequency recommendation?
3. Trap capacitor selection? To build a conventional (High Q) trap would
require a good HV mica transmitting capacitor. However, I can't find a
reasonably priced source for these old mica transmitting capacitors that
used to be so prevalent. One could use ceramic capacitors, I suppose.
However, according the ARRL Antenna Book ceramic capacitors are subject to
changes in capacitance when subjected temperature fluctuations. I live in
north-eastern Quebec where the temperature throughout the year can vary
between +/- 33 degrees C from zero. So, I don't know if I could use these
caps because so far, I can't find any charts showing the temperature affects
on ceramic capacitors.
4. Trap element lengths? None of the articles I have read provide a
formula or a rule of thumb for determining the estimated element length(s)
of the next lower band following the trap.
As my background is in the field of business and economics, any layman's
answers, suggestions or recommendations will be very much appreciated. Thank
you.
Bill VE2WMA
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