G7FEK antenna revisited.pdf

(392 KB) Pobierz
G7FEK antenna revisited
I recently came across a simple antenna design produced by Mike, G7FEK
An Ideal replacement for your 1/2 sized G5RV or Windom 40
Designed specifically for small gardens, it is an ideal replacement for those half size antennas
such as the 1/2 size G5RV and Windom 40 (51 ft and 66 ft long respectively). This smaller
antenna will give six bands, including
80 meters
as a resonant band with a full size 1/4 wave
element. On this antenna, tuning on 80 is not an issue and it will easily outperform the 1/2 sized
antennas by 10's of dB on this band.
•
•
•
•
•
•
•
Only 46 ft long x 24 ft high
Multiband operation
including 80 meters
without traps
Low angle of radiation is dominant on most bands (ideal for DX)
Enough High angle for short skip on 80m (local contacts)
Coax fed for convenience
Low visual impact
Ideal DX antenna for portable working
Details can be found at
http://www.g7fek.co.uk/news.php?page=G7FEK_Limited_space_anten_29507&embed=yes
A more in depth document explaining the construction can be found here.
http://www.g7fek.co.uk/software/G7FEK%20antenna.pdf
Another detailed analysis can be found here
http://www.rsars.org.uk/ELIBRARY/ANTENNAS%20DOCS/G7FEK%20LIMITED%20SPACE%
I was intrigued by the concept because the 450 ohm ladder line was being used as two parallel
Wires. These formed the vertical sections of two back to back, inverted L antennas for 80m &
40m. The wires at the bottom section of the 450 ohm feed line are connected together to provide
a common feed point. Which is fed against a counterpoise wire and the outer screen of the
coaxial cable.
My past experiences of antennas using 450 ohm or 300 ohm feed line in this way has been mixed.
The problem is that the spacing between the wire pairs is very small, which results in a great deal
of coupling between the wires. Almost to the extent that they could be considered to appear to be
a single wire. If the spacing is increased slightly this effect begins to diminish and at spacing’s of
greater than about 150mm the coupling is becoming negligible.
My initial conclusion that the antenna was basically just a single wire fed Windom (an antenna
which I had previously tried). So I produced a model using EZNEC which I based on a previous
model of a ZS6BKW (G5RV variant) antenna I already had, complete with 450 ohm feed line. So
it was relatively easy to modify this to look like the G7FEK.
I deliberately kept the radial system I had already got installed for my ZS6BKW in my model as I
hoped to be able to perform some A/B tests at a later stage.
However on inspecting the model it became immediately apparent that it was not a single wire fed
Windom and that it was indeed two back to back inverted L’s which were closely coupled by
means of the twin feedline, exactly as described by Mike, G7FEK.
This interaction presented some difficulties when I tried to get the EZNEC SWR plot to fit any
particular amateur bands. Any small changes to wire lengths made huge differences to the whole
plot. The SWR plot below shows my initial attempt using the dimensions given by G7FEK.
In order to try and make it easier to adjust each section I removed some wires from the model in
order to determine which wire was responsible for each dip in the SWR curve, and what the
various interactions between wires were caused by. This turned out to be a real nightmare to try
and optimise, as there are just too many variables to adjust, and too many interactions between
the various parts. It seemed that in a practical implementation you could get a low SWR on the
3.6 & 7 MHz bands relatively easily by adjusting the length of the two top sections. But obtaining
a low SWR on the other bands seemed to rely on having a large amount of luck (or having a good
antenna analyser and a great deal of spare time).
I also realised that the radial system was also interacting quite badly with the SWR curve. So I
removed the modelled earth radials and simple replaced them with a connection to the NEC
ground via a 30 ohm resistor. This produced much more stable results and I could now see the
two primary resonances of the twin inverted L sections, although there was still some interaction.
At this stage I decided to remodel the 450 ohm ladder line, by opening out the top end of the
ladder line between 0.5 to 2m wide (so that it formed a long V shape). I thought that this would be
relatively easy to implement in real life by using a short length of fishing line to ‘join’ the top ends
of the V together. It would also be possible to make the whole antenna from wire, which would be
cheaper and visually much less obtrusive than using 450 ohm ladder line.
Making this change immediately reduced the interaction between the main parts of the antenna
and made it much easier to optimise the length of each inverted L section. The longer one was
tuned to 3.6MHz and the shorter one tuned to 7.1MHz. I finally settled on a 1m spacing as being
the most practical to implement.
The next step was to try and emulate the counterpoise wires described by G7FEK which he ran
underneath the flat top section of the antenna.
I decided that the most likely scenario would be to run the wires along a garden boundary fence
(as was the case with my ZS6BKW) so I added some raised counterpoise wires at 2m above
ground level, whilst still retaining the NEC ground connection via a 50 ohm series resistor
In order to achieve this I had to raise the overall height of the antenna by 2m, but I felt that having
the horizontal top section at 10m above ground would be typical of many instillations.
I also experimented with changes in the value of the resistor I had connected in series with the
ground wire. As mentioned in the G7FEK document a good SWR figure at 3.6 & 7.1MHz was
dependent upon having some degree of ground resistance, but providing it was between about
10 and 40 ohms the match was reasonably good. As this is typical value of resistance achieved
with a modest small radial system, it seemed to concur with G7FEK’s observations.
These changes further improved the SWR curve, and after a few more iterations of counterpoise
wire lengths, I was able to get something close to the SWR curve as shown by G7FEK, except
on 21MHz.
To resolve this I followed G7FEK’s suggestion of adding an additional wire about 4m long, which I
tuned to 21MHz, but I placed it in the centre of the V feed, as in practice it would be quite easy to
support this additional vertical wire with another section of fishing line.
Here’s the final arrangement.
The resistor emulating the ground resistance of the radial system is shown as a small square,
and the 50 ohm feed point is shown as a circle. The coax cable inner goes to the bottom of the V
and vertical wire.
The coax screen connects to the counterpoise wires at 2m above ground and the radial system at
ground level. This can either be via a connection directly to the screen of the coax at ground level,
or by a separate 2m long wire or strap. The radial system should consist of at the very least of
one ground spike. Ideally it would be a minimum of 4 to 8 buried radial wires (beyond this number
it’s a lot more work with not much return) of about 10 to 15m long. If the radials are shorter than
10m need to bury more of them.

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika:

Zgłoś jeśli naruszono regulamin