G7FEK Limited Space Antenna: Part 3 - 80M DX

Now it's time for some 80m DX with the G7FEK limited space antenna. Here I work Phil - W6UC.

All you need is a cheap fiberglass pole, go up about 7.5 meters and then across another 12 meters or so depending on tuning :) The wire can go into near by low trees or other fiberglass poles.

This antenna basically is quarter wave inverted L's on 40 and 80 meters. Full-sized quarter wave elements on 30 and 20 meters.

It all comes off the one feedline. Impedance allows for automatic antenna switching.

G7FEK website: http://www.g7fek.co.uk/blogus/newsshow.php?page=80m_Antenna_for_small_gar_49493

G7FEK detailed PDF: http://www.sral.fi/turva/G7FEKantenna.pdf

Some additional notes sent to me by Mike G7FEK

Hi Andrew. Great to see another build of my antenna, great job, more thorough than my usual portable lash ups!!

There's a couple of points Id like to share, just for the benefit of other constructors who see your video....

1) The extra element you have for 30m is not normally needed as the top is already a resonant half wave near end fed OCF, at or just below 10MHz, and is matched via 1/4 wave transformation effect of the vertical section (like the ZEPP). A counterpoise just shorter than a quarterwave on 10MHz (about 7m) is usually sufficient to get this band working well on 30, although an ATU is usually advisable to null out any inductive reactance present (there will be some). I think you needed this extra element because you have the antenna in a corner configuration with a very acute angle. This a great space saving idea and teh antenna works well in the corner of a plot, but narrower than 90 degrees, this effectively shortens the electrical length of the top on 10MHz by folding the top back on itself. It will affect 30m to some extent, but it should work fine on most other bands though.

2) The feedpoint does not have to be elevated. It can be at ground level. Trying to find that extra bit of height might be a problem for some people, so I just thought I'd point out that it is not necessary to elevate the feedpoint, raising the far ends of the radials for at least the last few meters is essential though. Radials should be insulated wire (2.5mm2 electrical wire works OK, but thicker is better). Other ground systems can be used alongside, or instead - even a simple ground rod if soil is conductive enough.

3) The minimum number of radials is two (one for 3.7 MHz and one for 7.2 MHz). This will get most bands working and probably all bands if you use ATU to trim reactance. Others radials are optional, but improve performance and matching for sure.

4) I recommend a choke at the feedpoint, especially if you skimp on the ground system. The RF return current will take the easiest path to ground, even if it is via your radio and house supply!. You can use the coax outer as an extra radial by placing the choke at 1/4 wave away from the feedpoint on the chosen band. A simple and effective choke is 10 turns of RG58 on FT240-43 toroid. 22ft of RG213 wound around a plastic drainpipe works very well.

5) I love the idea of the high inductance DC shunt to remove any static charge. This will also offer some protection for the radio front end if you forget to unplug it before a thunderstorm. I'll start recommending this to other users and give you a credit!. Static isn;t usually a great problem at 24ft, but it will certainly help.

All in all, almost everyone who has tried the antenna has had great results in getting almost all amateur bands includign 80m. The four main ones without needing ATU, in a very small space and without the usual extreme compromise on 80m. That's what it was created for and 80m is an essential band in sunspot minimum!!. You always get some jerk who will say it doesn't work multiband or on 80 because they "modelled" it or whatever, but the original was built from pen and paper and tested in the real world by hundreds of Hams. Enjoy.

As you are doing so well with local contacts, DX on 80 should surprise you even more as the peak angle of radiation is quite low, towards the horizon on all bands. It can often beat a full size dipole at same height for DX on 80. Looking forward to your nighttime DX video, lets hope for a good opening!!.

All the best. Mike. G7FEK.

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