Centerloaded mobile antenna - Handy for stationary /M & /P
The higher up the antenna you load it, the more efficient it will be!.
I have always wanted a mobile antenna with resonable effiency that i could use
when parked at the seaside for DX'ing from the car. I wanted something i could
easily put on the roof on the magnetmount i have on permanently.
Countless experiments where i live showed that it's not practical to have anything
longer than 5 meters on the roof. But thats still WAY better than the normal mobile
antennas you can buy which range from 1.5 to around 2.5 meters. The longer
antenna element raises the resitive component of the antenna, and thus also
the effiency. And the center loading pushes the current maximum higher up
where the losses are smaller. So i set about trying to build something that
would work for me.
I decided to build a center loaded setup with exchangeable loading coils.
Total lenght 5 meters, so two halves each 2.5 meters long.
i decided to make it in 4 sections like this.
Bottom section 1.25 meters long, from 12mm aluminium tubing
First mid section 1.25 meters long, from 10mm aluminium tubing
Second mid section 1.25 meters long, also from 10mm alu tubing
Top section 1.25 meters long, from 6mm aluminium tubing
Then i made small inserts that would fit halfway inside 2 pieces of tubing so i could
join the 2 halves, securing them with hoseclamps. This worked pretty good.
i did this for all 4 sections, and made a slit in the bottom that would allow it
to slide over a stud on the antenna mount, again being secured with a hoseclamp.
Time to make the first loading coil. I used Eznec to simulate the antenna and get me
close to the values i needed to make a set of coils for 40, 80, and 160m.
My idea was to also make an adapter that could go in instead of a coil so i
could use the full antenna on the higher bands via a tuner i have in the car.
Here's how i did it.. there are many ways of doing this. this is just what i did.
I did my calculations based on a 32mm coil former. PVC pipe is cheap and so are the
end caps for it. So that was my model. A piece of pvc pipe with the coil itself, and
2 end caps with holes for 2 pieces of alu tube that would connect the coil to
the rest of the antenna.
I cut 2 pieces of 10mm aluminum tubing which was 8mm inside diameter.
i had some surplus fiberglass tentpoles, 8mm thick, which fits nicely inside the alu.
Each alu tube is 100mm long. Then i slid in the fiberglass piece and made sure that there
was enough space between the alu so that they would not be inside the coil when finished.
I put glue on the fiberglass and left it to set, then i drilled and tapped two M3 holes
and fitted two solderlugs for securing the coil wire to the aluminium tubing.
I then made the loading coil for 40m, just about 16uH was needed for my setup so about
22 turns on my coil former. Measured with an LC bridge and windings spaced to give the
value needed. I used a dab of contact cement to keep the windings in place.
Just another image of the coil
I then soldered the wire to the solderlugs i mounted earlier, securing the coil to the
aluminium/fiberglass bar. Then it's just a matter of drilling 2 holes in the PVC
end caps and slide them over the tubing. I glued mine to give it some resistance
to water and add mechanical stability. Theres a lot of flex in a 5 meter antenna.
My finished product looked like this. Centered so that the alu tube is eaqually inside the
coil former on each side. it's really important that the alu tube does not reach inside
the coil itself. this will throw of the inductance and give all sorts of problems.
Closeup of the inside of the coil. it's difficult to see exactly whats going on but it might give
you an idea of what it looked like. Maybe you know of a better way to do this ?
(im all ears :-)
This is what my finished 80m coil looks like. Drilled two 10mm holes in the end caps
and slid them over the tubing. Sealed everything of with some heat shrink tubing.
And this is how the loading coils connect to the rest of the antenna. Remember my two
mid sections was from 10mm aluminium tubing so how will this fit if the two pieces of
tubing on the coil assembly is also 10mm ?, well i made the piece that joins the sections
from 12mm tubing, with 10mm inside diameter :-) so everything goes together nicely.
After testing the antenna with the different coils i made the adapter that allows me to use
it on the higher bands via the ATU i have in the trunk of my car. It works great.
Being 5 meters long it's possible to use it on 20m without anything else as it is
a 1/4 wave long on 20, so acts like a normal GP antenna.
This is the adapter i made to connect the antenna to the normal PL socket on the roof.
It's basicly just a PL to 3/8'' adapter, a heavy duty spring with a 3/8'' stud to the right
that allows it to be screwed into the adapter. Then i tapped the hole at the top of the
spring (left) to 10mm metric. I then drilled a small M10 bolt and tapped a M8 hole in
the middle of it. i had a long stainless M8 bolt which i screwed in and cut the head off.
I beefed up the lower end of the antenna by inserting a 30cm pieace of 10mm alu
tubing to stiffen it up, and that 10mm piece has an inner diamter of 8mm which
fits exactly over the mounting stud to the left.
I ended up with a contraption that connects to the PL socket on the right, and the
antenna will slide over the stud to the left, being secured with a hoseclamp.
Works great and the spring takes a bit of strain form the PL socket on the roof of the car.
I have tested the setup with all the loading coils and it works great when parked
at the ocean. It certainly is a lot better than the "normal" mobile antennas i have
tried... Give it a go, you will not regret the effort you put into it :-)