Multiband portable wire antennas - High efficiency on the go (Updated)
As you might have guessed by now, antennas are my great passion.
on this page i will describe a few of the ones i use while portable.
There are so many designs to choose from, but a few stand out
with better performance and lower take off angle than most.
One of the key aspects in my portable operations are the type of
antennas i work with. Sure the easiest way for all-band /P is 13
meters of wire with a tuner at the bottom, but this approach is
flawed im my mind, due to the need of a very substantial ground plane
and the very high take-off angle your signal will have on the high bands.
Radials, radials... i hate them :-) they are bothersome at best and
not my idea of a good time trying to lay down 12 or 16 just to get
reasonable results. In my mind the end fed 1/2 wave antenne is
the way to go.
As the feed point impedance on half waves are very high, so is
the radiation resistance, and thus, no elaborate ground system needed.
In turn you get VERY high efficiency because you have no ground loss.
Often the shield of the return coax is plenty enough, and because the
current to ground is so small, you wont have any problems with
common mode currents creeping up you coax either.. pure bliss :-)
In the below picture theres 2 different versions of exactly that kind of
antenna. On top of that, these have an inline inductor which will make
the wire half a wavelength on at least 2 of the design frequencies.
The image is pretty much self-explanatory.
The top one is the one im using for portable op's right now and it works great.
When modelled with EZNEC it gives a take off angle of around 19-25 degrees
on the bands of most interest to me, and less than 2:1 SWR, a very healthy design.
This is the 110uH inductor i use for my antenna. wound by hand geez what a job
but it works like a charm. i have used some heat shrink tubing to seal it of.
To feed these antennas you need a matching device. Heres one described in
one of my other project pages, wich i have wound on a small T140-43 core
good for a 100 watts or so. You can use similar cores without problems.
You will have to solder a 100-200pF capacitor across the input to obtain
a good match on the higher bands, like 10m.
This is my finished product, and the one i use today when portable.
I have put my 34uH coil in a small plastic bottle to keep snow out.
It will run on 40-20-10 without tuner, and 12-15-17 with an ATU.
The image below shows a more simple (GP) dualband design for 40-80 meters operation.
This is how i use the setup with my current car. i had to come up with
another way of rising and holding the mast when i got the smaller car.
So i came up with a "drive-on-mount" which seems to be doing pretty
good, even in strong winds.
Closeup of the antenna mount. Simple but effective. the bottom
bronze color thing is hinged so i slide the mast bottom in place
and walk it up, before securing it with the top bracket.
UPDATE July 5th 2015
So once again i sat around pondering about antenna design, and especially
antennas for portable work. Allthough the halfwave antennas described above
are working very well, it is still a hassle to change bands. So i set out to make
an all band antenna for portable DX work and for my Flora-Fauna activatons.
The theory behind the idea i had was that i could use 10-12m of wire secured
like usual to a glasfibre pole. Then i could electrically divide that length of wire
into 2 pieces using a good quality trap, so that only the lower 2/3 would be
used on the higher bands, and the whole length on the low bands like 80&40m.
This would also provide a relative high feed point impedance on all higher bands
compared to a normal GP antenna.
Resonant ? no, reactive ? yes.. thats what the tuner is for ;-)
This way i can make sure that my take-off angle on the high bands is kept in
check. The normal way to do this is to divide the antennas length into 1/4 wave
sections with traps for the appropriate bands, but the take-off angle with a
1/4 wave antenna is around 25-30 degrees. Not unusable but not good either.
So my idea was to divide the 10.5 meter antenna wire into 2 sections, the
lower part being 7.5 meters, and the top part being 3 meters long.
The setup will use 8 elevated radials, and be fed through a tuner at
the bottom of the antenna.
I ran the design through EZNEC and it looks like a healthy idea. Of course
i built a model in the backyard to try it out for real and compare real-life
with the simulation results. I must say they were very coherent. The
simulation showed that the antenna would have an imaginary part that
would be inductive on all bands (above 40m) except 15m where it would
be capacitive in the vicinity of about 6-700 ohm. Easily correctable.
The real life tests and measurements showed this to be correct, and it was
just capacitive enough that my tuner could not reign it in, so a small loading
coil was made to cancel out a portion of the capacitance, making the tuner able
to handle the load, without adding too much inductance to the other bands.
The trap is 30pF & 4.2uH , the loading coil is 2uH, both wound on 32mm PVC.
So the final design ended up like this:
The antenna will work on 80 with OK results, it will work fine on 40m, and it will
be a good deal better than a normal 1/4 wave GP on 20, 17, 15 meters.
I tested out the antenna yesterday July 4th 2015 with mediocre HF propagation on
most bands and the antenna did very well. i raked up 160+ contacts in the 3 hour
activation of ozff-131 so that was pretty darn good all things considered.
The antenna really got tested during last weekends IARU HF Championships 2015
the amount of contacts was not that great due to propagation, but the
contacts i worked says that my theory and EZNEC predictions was pretty healthy.
The takeoff angle and the radiation effiency cant be that bad afterall :-)
China, Khazakstan, Hong Kong, Chile, Brazil, Argentina, Puerto Rico on 15 & 20m
40m was about 1-2 S-points less than my fullsize dipole but the reduced sensitivity
is not always a bad thing because it also means less noise. And although not
being as sensitive as the dipole, it is my impression that the signal to noise
ratio was actually better on the vertical during the contest, apart from the
middle of the night, when it's dark the dipole reigns supreeme.
Heres the trap i used for testing, capacitor made from RG316 teflon coax
(i used silver-mica for the final edition, no difference detected)
Heres the RG316 coax capacitor tugged inside the trap
The small 2uH loading coil
just a snapshot of the groundplane i used
Thanks for looking :-)