No room for a 160m antenna ? - try this one on for size          


Top-band is not out of reach, even if you only have a small garden.

What to do, what to do.. the sun is taking a break

As the current cycle is on fast decline, something must be done if we want to continue having
fun at the radio. One option is to go for the low frequency bands as they suffer less from
the lack of sunlight (pun intended).

I've just overhauled my W3DZZ and raised the apex a couple of meters so im in ok shape
on 40 & 80 meters. But theres one band i have always avoided because of space
restrictions, and thats the gentlemans band, 160 meters.

A lot of different small or electrically shortned antenna models are available but they
all have low radiation resistance and a lot of reactance, neither is a good thing
for effiency or matching the antenna to our 50 ohm coax feeders.

My garden is on only 225 square meters so i have no room for the normal 1/4 wave radials
and i dont want a horisontal antenna on such a low frequency. So i came up with this
solution which, i must say, is not that bad when you consider the small footprint.

I started out with the groundplane. I made it as good as possible in the small space
available, so 18 radials as long as possible from the center of the garden and outwards.
These are all connected to a ground rod and so is my normal antenna tower
for safety reasons like lightning and so on.

I cut grooves into the lawn with a electric lawn edge cutter (100 EUR)
If you have lots of radials i can really recommend this labor saving investment.





I had some square aluminium tubing in my shed. 40 x 40mm wide and 2m long.
So i made a vertical radiator, 12 meters tall from them. Then i ran some 14m
horisontal antenna wire from the top of the alu pole, to a fibreglass mast
at the other corner of the garden. Then the wire continues down the fibreglass
mast and ends 1 meter above ground level. So basically a square inverted U

This is the far end of the antenna. I've not seen any corona or arcing yet!.





This configuration gave, acording to my countless models, the lowest reactance
and highest radiation resistance possible, and only 14 meters long in the H plane.

All the radials are interconnected in this box, together with the ground rod and
station earth. My antenna switchbox grounds all unused antenna inputs for safety.





The feedpoint is housed in this box beneath the vertical radiator.
I ended up with a feedpoint impedance of ~30 ohms, which is nowhere near true
but the analyzer does not know that. It sees the total resistance of the circuit.
My models show that the true R is somewhere around 7-8 ohms, so the rest is
loss in my not so great groundplane. But that is to be expected with such limited
space for radials. I could have used elevated radials, but that is not practical for me.

I used these brackets for mounting, and isolated the radiator with thick rubber mat.




To bring the antenna to resonance at ~1925 khz i only needed a 4uH loading coil
in series with the antenna and that is placed in this box with the coax feeder and
ground connections. You can use the wirelength that will fit your lot, the only thing
that will differ from my setup is the matching.

Too short* you need series inductance and too long*, you will need series capacitance.
* Compared to a electrically 1/4 wavelength for any given frequency.





Heres an inside view of the box at initial tuneup, with the loading coil and the static
bleed off inductor. I know you can buy ready made bleed off inductors but come on,
it will take all of 10 minutes to make one yourself :-) i had some 30mm fibreglass
tubing so i wound as much wire on it as i had room for. That gave a final inductor in
the range of 100+ uH. Thats more than a kilo ohm resistance at 1.9 mHz..for free.

8 windings of 1.8 mm magnetwire on a 40mm pvc former provided the 4uH loading coil.





The sweep after final matching looks like this.
Notice the very wide bandwidth for such a small antenna. That is again due to
loss in the groundplane. If i were to place more and longer radials the bandwidth
would narrow and the SWR would go up. But thats all part of the compromise :-)





This is the EZNEC model, it's rather simple but it was the best setup for the space
available. Resonance does shift a bit when wet or dry and it is listening pretty darn
good compared to what i have had before. It even has some theoretical gain ;-)






Now all that remains is to test the antenna out. i will update this page
with my findings. But no matter what, it got me on top-band ! 

:-)


Update March 4 2017

 Just completed the CQWW 160M SSB contest and i think the antenna has proven itself.
On RX the normal noise level on the antenna at around 18:00 is 2-3 S units and
dropping to zero in the middle of the night. not bad for 160M DX'ing in suburbia

I worked 213 QSO's in total incl. Kasakhstan, Greece, Cypress, Turkey and Canada
(100w) so despite a compromised design, it worked quite well from this small lot.

Heres a map of my contacts during the contest, click image to open larger.





Next step is to add a massive choke to the feedpoint ( 11 turns H155 on 3 stacked FT240-31 )
This will require me to match the impedance all over again, but will keep the shifting to a min.
It will also allow me to run 1kw into the antenna without problems with RF feedback as
it will isolate the antenna completely from the feedline.

The original design has no problems at 100w.



Update March 12 2017

After placing the choke mentioned above at the feedoint, the resonance shifted as
expected to a lower freq. So some additional matching was required. The choke
was placed indoors at the shed around 10 feet from the actual feedpoint.





This was done by adding 360pF in parallel with the feed to cancel out some of
the inductive reactance of the antenna. In fact this will electrically shorten the ant.
From left to right: ~5uH loading coil, 3x120pF, 100uH static bleed off coil.
(the bleed off coil is not connected to the caps, the image is skewed)

So to recap, if your resonance point is too high, your antenna is electrically too
short, and you can add a loading coil in series with the coax feed to lengthen it.
If your resonance point is too low, your antenna is electrically too long, and you
can add capacitance across the feedpoint to shorten it.

There are many ways to do this matching, this was just the method i chose
your milage may vary :-)





To minimize the influence of the groundplanes effect on my feedline further, i decided to
cover the coax feeder with some of the protective metal covering used by electricians
for outdoor cables. So this is what the final assembly looks like.





Next step is to do some testing with higher power and observe how the resonance
behaves with summer and less rain approaching.. i cant wait ! :-)

If you have any questions please click
here and i will answer as soon as possible.
Choose "sign guestbook" to contact me.

I hope i have inspired some of the sceptics out there to give the gentleman's band
a try. Even if room to play with antennas is limited, ingenuity will get you through.





DX