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28 and 24 MHz Limited Space
3/4 Wave Delta
Loop
(With
added modification for 360 degree coverage)
by
KL7JR
Here is a simple and easy to build antenna in a Delta Loop
configeration that should make 10 and 12 meters lots more fun if you
have a limited space situation or you just like the idea of a smaller than
normal sized Delta Loop. Gets lots of DX too and there is a simple
modification using one boom and 2 Delta Loops for complete 360 degree
coverage!
John tells us how he did it....read on!
For winter
portable DXing up north, I needed a compact, simple to install antenna for
both 10 and 12 meters that offered directivity and was economical to
build.
I decided on a ¾ wavelength delta loop made from PVC pipe,
(the backbone), and #14 gauge wire.
I reduced the size of the loop for
ease of installation reasons (not because of space limitations) from
previous experience of erecting antennas when the temperature was forty
below!
Using the standard formula for loops, 1005 divided by
frequency in MHz gives us the one-wavelength design length for a
particular band.
Now, what should I use for the design frequency
when I want to use two different bands? Easy, just split the frequency
difference like this:
28.500 minus 24.950= 3.55 MHz divided by 2 gives
us 1.78 MHz.
Add 1.78 MHz to 24.950 (or deduct 1.78 MHz from
28.500) gives us 26.73 MHz "center" design frequency.
Now take
1005 divided by 26.73 and you get 37.60 feet times .75
(for ¾ wavelength size)= 28.2 feet
long.
Divide that number by 3 to get the length of each side
of the delta loop gives us about 9 feet and 5 inches long each side.
A balun is not required but a good
antenna tuner is for operating both bands.
You’ll be amazed at
the performance of this antenna especially for what it costs to build.
Get it up as high as you can and have fun on ten and twelve
meters.
I worked a lot of DX from Alaska and the Yukon switching
back and forth using a pair of these wire beauties (one facing north/south
and the other east/west) at only 20 feet in the air! (See mod below for
dual loop version)
Just remembered
that I also used this antenna “indoor” for 2004 ARRL 10 Meter Contest when
I was portable, ( Long Island ,
NY ), from a 3 story brick apartment complex.
Antenna was not in
a perfect delta configuration and duct taped to a
sheet rocked wall on an end apartment unit. It was easy to work a lot of
DX even before and after the contest on 10 &
12M!
I’m
sure I caused a lot of tvi, hi hi!
P.S. I also used this design, (cut for
10M), at my Washington state QTH hanging off my tower at about 30
ft.
It was a good performer there
too!
The "Mod" - Taking
it further!
Dual 5/8 Wave Delta Loops on a
single boom for Omni Operation!
Let’s take the "28 and 24 MHz Limited
Space Delta Loop" design a step further.
Say you want two separate
loops in opposite directions sharing the same "boom" (and don’t mind going
from ¾ WL to 5/8 WL) and you only want one simple support for the antennas
for 360 degree coverage. Asking too much? I don’t think so.
The 10 and
12 meter design frequency (ref. above design on Hamuniverse.com) is 26.73
MHz, therefore 1005 divided by 26.73 gives us 37.6 feet times .63 (5/8
WL)= 23.69 ft.
Call it 24 feet for simplicity divided by 3 is 8
feet long each leg of one delta loop antenna.
Don’t stop here,
lets shorten the top legs of each delta loop (we’re building two,
remember?) by 25% to better fit on the shared boom (or PVC support frame).
The tops of the loops are now 6 feet long, therefore each side must be
9 feet long to maintain the total length of 24 feet. It’s a bit odd
looking but should work just fine according to my experience with loops.
Each loop will look something like the Side View drawing in the
picture below:
Above
Drawings not to scale.
Now, let’s work on the
PVC support frame. It’s basically a "T" like the Bird's Eye View in
the above drawing and includes rope supports from the top. Each loop
is suspended from each "part" of the "T" boom facing different directions
and 90 degrees from the other. Wires are hanging down towards the
ground.....or can be tilted slightly from the "boom" at an angle to
suit your height requirements at your location.
"Dots" on the boom in the
drawing above are for rope slings to be top supported. Make the
"A" PVC section 6 feet and 2" long and "B" PVC
section about 7 feet and 6 inches long, (for a little added space
between the antennas).
I’d use 1 ¼" PVC Schedule 40 with one "T"
fitting centered on the support.
You’ll need
separate feed lines for each antenna and an antenna switch plus a good
tuner.
Total price of the dual antennas should be about half
what you’d pay for a good CB vertical provided you have an extra feed line
and antenna switch not in use.
The feeling of accomplishment of
building it yourself will be priceless!
Although I haven’t tried this
design yet, but for those who do, you may want to offset the "T" fitting a
bit more if you experience interaction between the loops. Good luck and
have fun on 10 and 12 meters!
Now you will have a Delta loop for
North/South or East/West coverage depending on how you install the antenna
at the QTH. No matter how it is installed you should get a full 360
degree coverage!
73, "Yukon John", KL7JR
John F. Reisenauer,
Jr.
Editor's Note:
There are several ways to support or suspend this antenna whether
it is a single or double loop. Aside from getting it as high as possible,
you also want to prevent it from twisting or "flying" in windy
conditions.
Just by chance that you do not have
a way to support the Delta loop from the top, you could find the
center balance point of the "boom" on each style of
antenna in the above article and use that point on the boom for
supporting on top of a PVC mast.
It is advisable to NOT
use a metal mast "inside" the loop. It could interact with the
antenna if used.
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