Simple Loop Antennas
by G0FTD
Updated 5 Nov 2011
Several years ago I wrote a short article about my experiences in using very simple
wire loop antennas for discrete use. I have now decided to give it a little
update.
It all started when I found myself away from home and often abroad through
my employment and wishing to use some of my amateur radio equipment
from the various hotels or bed and breakfasts I stayed at.
My employment WAS in designing antennas afterall, so it was about time
I used the experience for my own requirements!
My experience has shown if you have to use an indoor antenna it's probably
best to use some kind of loop as opposed to a simple vertical or end fed
system. Not sure why, but it just seems to be the case, especially when
trying to use the LF bands. I have also found a preference for making loops
standing vertical as opposed to horizontal.
Loop Version 1
This simple loop antenna was in use by me for several years in various
QTH's. You start by taking a basic square loop of approximatley 2.4m
long for each side. You feed it at the bottom centre, via a 1:1 balun
and a T-Match ATU.
THE 1:1 balun I found to be of great assistance when using extremely compact
stations in reducing noise on receive and RF flowing other station cabling.
The latter sometimes manifests itself with RF bites when touching equipment,
varying SWR when touching equipment, or RF on cables connecting PC equipment
to radio equipment. WARNING! This is a common problem with all stations or
unbalanced antenna systems. I've damaged computer equipment in the past, so
ALWAYS do some basic checks first before applying more than about 10 watts.
You will note that I also specified a T Match ATU. A T Match will provide
a much greater range of impedance matching as opposed to something like
a Pi or L match system.
Both the balun and ATU MUST be used actually at the feed point of the
loop, and never ever at the rig end. If you try the latter you will get
a reduction in performance.
At the top centre of the loop you may put a simple open/close switch.
This can be used on 10/12m to alter the polar pattern by 90 degrees.
In other words, you are able to rotate the beam of the antenna around without
actually moving the antenna! It also switches that radiation from vertical
to horizontal.
This loop can be used from 160m - 6m. I have rarely used more than 15w
and had massive success especially from 30m upwards.
What have I worked ?
The world! Right down to VK land, South Africa and South America.
On 6m I've never used more than 5w and I seem to work just as many
stations as anyone else during the summer sporadic E season.
On the LF bands performance is reduced, but I have worked the
Ukraine on 80m, Cyprus on WSPR and regular QSO's on 3.560Mhz CW
on the QRP frequency at night.
I've made a few local QSO's on 160m,about 30 miles away but I've never
made too much effort on that band using this small loop.
Loop Version 2
In an attempt to improve LF performance I tried a large loop.
This was approximately 4.2m x 2.4m and fed in the corner. Again the use of a balun and T Match atu is essential. This seemed to give a useful performance boost on the LF bands, and works well up to 6m.
The radiation pattern is pretty omnidirectional at LF, moving to bi-directional on the upper HF bands, and then on 6m it becomes a four lobed system offering about 2db gain over a dipole.
Using an Indoor Station
I originally wanted to use these loops purely for indoor use,
and I've had a lot of fun with them over the years.
Naturally you cannot expect to be as big a signal as others,
but that doesn't stop you working DX.
I usually find CW scores over SSB, and now there's modes like PSK31
and WSPR. The latter mode is an indoor QRP'ers delight and I thoroughly
recommend it.
So if you are a flat dweller, have no garden or live in a broom cupboard
this proves that the HF bands ARE available to you.
So now the sunspots are up - get cracking and have some fun.
73 de
Andy G0FTD
|
