As I
live in a small, old house in a conservation area, I am rather restricted
with regard to antenna's, in order to operate on the hf bands I
have overcome this in two ways, first, with my simple end fed antenna,
and secondly, inspired by Will/GM0HKS I have made a Mag Loop antenna
based around a wonderful vacuum variable capacitor which has winged
its way from a Russian military maintenance depot via the Ukraine
by way of ebay!!!
It cost
only £22 with £10 delivery and was duly delivered recorded
delivery 6 days later, not bad from the Ukraine.
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I purchased 2 - 3 meter lengths of 15mm copper water pipe -----
problem number one, how to bend the pipe. I made a former out of
MDF, which is simply a small piece filed in an arc and screwed to
a larger piece of MDF - as in the illustration above. I also made
an arc with a pencil and some string, to represent the final circumference.
With nothing more than my thumbs and stomach muscles, a bending
spring is not necessary, I managed to get the two pieces into a
circle - not perfect but near enough.
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Now I have the copper bent I have to think of some way to support
the loop. I came up with the idea of using white plastic drain pipe
and fittings. As can be seen by the photo I used a 90 degree bend
to support the copper loop. At the bottom end I simply made a support
using old wood and another price of plastic pipe which was conveniently
a little larger to enable the support to "plug in".
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The capacitor is attached to the stand using 2 brackets cut from
discarded copper pipe, a small piece of wood and 2 nylon bolts liberated
from a toilet seat!!!!
As you can see by this picture, the capacitor is connected to the
main loop by once again using off-cut copper pipe cut vertically
and opened up. Yes, the copper can be cut with nothing more than
a cheap pair of scissors.
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The Gamma Match is made from 1/8" copper tube (this is used
by fridge engineers) connected by small copper brackets. These brackets
(as with the top ones) can be moved to enable adjustment and if
necessary soldered at a later date.The cable is connected to a PL239
socket.
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The finished loop (with my car to give an idea of scale). Tuning
of the loop is done with a piece of wooden dowel and so far it seems
to tune easily to the intended 40 meter band.
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Spec---
100 pica farad vacuum variable capacitor
Diameter - 59 inches (circumference 15.5 feet approx)
Gamma loop - 37.2 inches
Made from 15mm copper water pipe (2 - 3 meter lengths - Focus DIY)
and 1/8th inch copper pipe (donated by my colleagues at work)
The wood and plastic drain pipe, rescued from skips.
UPDATE
MAR 2011
Ok, Loop
performed well on 40 meters and above but in order to tune to 80
meters an additional capacitor is required. A 400 pica farad vacuum
variable is horrendously expensive so I have made a simple home
brew affair to "piggy back" the vacuum variable
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The vacuum
variable is now a trimmer - works great!!
As I am now looking to portable operation I need to disassemble
the loop to fit into my new (old) Micra. First of all the yorkshire
fitting at the bottom was removed and replaced by a simple compression
joint (£2-30-Plumb Center). Now the gamma match, two problems,
first it will not give me a 1 to 1 on all bands and secondly it
is too delicate (the solder joint on the PL239 socket is vulnerable).
I have experimented with a transformer match but it will only work
on the design frequency. I have also made a couple of faraday loops
but they also will only work on the design frequency. Solution,
an adjustable gamma match.....
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The coax
is secured to a plastic bracket, center to the match and braid to
the loop, the bracket at the other end can move up and down as it
is secured by only two wing nuts.
With
this moveable match a 1 to 1 can be achieved on 80m to 20m
Don't
forget, 15mm copper pipe can be bent with only your hands, the pipe
can be cut with sissors to provide brackets and capacitors can be
home made.
Health
and safety ----- high voltages can, sorry, will occur around the
capacitor!!!
Hope to qso soon HF CW ---------73
Dennis MØMMX
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