Monthly Archives: August 2013
The Vertical Dipole

I plan for a while to try out another vertical concept, since I figured out,
that my 6.5 Meter Vertikal having some trouble coming to its full performance
because my Truck roof is definately not enough for Bands below 17 Meters.

The Question of the right radials on a multiband vertical is a good question,
it should be $frac{1}{4} lambda$ most of the designs simply add verticals
for each band to the design. You could also use an earth tuner to match your
antenna to your vehicle, note to me: schedule some experiments with earth tuners
on the sidecar radio outfit.

An other way is to look how dipoles are tuned and adapt it to a groundplane
antenna. So you can use some radials with exactly the same length as
the antenna, and than feed it with a balancer unit. I did it with my antenna,
and it worked almost well, almost because the 1:1 voltage Balun I used maybe
was not the best choice, but it worked anyway. The main problem I had with the
groundplane design where the radials. 6.5 Meter to every direction means lots
of extra space around my truck, I can’t reduce length with couls because
the coilsize debends on the frequency. And the only Idea I had for the
mechanic of the radials was the use of tensioned wire.

So I need a design which is playing without radials! A vertical
Dipole for example. With a feed in the middle. There is a vertical
$frac{1}{2} lambda$ monoband design available in Rothammels
Antenna book which feeds the antenna with koax cable right through the bottom
dipole element. The design is basically the same as with a normal dipole exept
that it is vertical and the feed is not coming from the side. So should be
capeable of multiband work by adding some sort of Balun some sort of
Tuning to the feed point.

This raises to questions: How to tune, and how to do the Mechanics.
The Mechanics question was solved by ordering a Havy Duty 10 M GFK
Mast at dx-wire.de. the mast costs about 70 € but alows rapid
development of different antenna designs with much cheaper materials
than the aluminium tubing, I used since then. I can now use old RG213
Coax cable or speaker cable as antenna element for example. I also ordered
the portable 10m version from dx-wire which soon will improve
my portable antenna situation soon.

The next problem to solve was the matching problem. I want something
which I can control from the shack. It is possible to place a symetric
autotuner at the feed point i decided against to make the feedpoint
simpler and lighter. If you match in the schack you have higher losses
on the Koax cable. The choice which death to die was finally made by the
large collection of available Unsymetric autotuners, and the time i had available.
I can still add some extra antenna tuning in the next project, when I know how the
whole thing plays. Ham Radio is still an experimental Service, so what.

I build an antenna with two 3,65 m long elements. This length should work from
10-15 Meters. it is a little bit to short for 17-20 and 20 Meters. At 40 meters
there is still a little less than $frac{1}{4}lambda$ left, but a single element
is to short. On 80 Meters such an antenna will be some sort of wet towel, but still
better (longer) than my other 80m solutions.

image{IMG_20130801_220331.jpg}
image{IMG_20130801_220355.jpg}

I decided to use RG213 cable for the upper element. The schielding radiates the
waves the inner conductor is left unconnected. The case of the Feed is made of
waste pipe and sealed cable feedthroughs.

image{IMG_20130801_205416.jpg}

The lower antenna element is a bit more tricky, I stripped 3.65 m copper schielding
tube over the ecoflex feed cable and isolated it with shrinking tube:

image{IMG_20130801_222027.jpg}
image{IMG_20130801_224536.jpg}

The next part the (first) balun. A 1:1 Voltage balun. 8 Windings per Coil
50mm waste Pipe.

image{IMG_20130802_211539.jpg}
image{IMG_20130802_211539.jpg}

The whole antenna setup:

image{IMG_20130803_003955.jpg}
image{IMG_20130803_114921.jpg}

Now to the Problems did start:

I could match the Antenna only with a Manual MFJ T-Match not with the Autotuner I want to use.

  • With 1:1 Voltage Balun Fail 40 Meter
  • With 1:2 Voltage Balun Fail 15 and 80 Meter some bands not optimal
  • With 1:5 Voltage Balun Fail on 80 Meter and some bands not optimal

Then I tried a Current Balun:

From 10-40 Meter best match, 80 Meter is not working. So I tried out to add the Coil of the manual
L-Matchbox which I build 1.5 years ago. The SWR indicator is moving, so I need more inductivity.
I killed an old CB-SWR Meter (I have lots of them) and found an old Ferrit ring in my shack, and
made a coil. I build in a Switch that plugs in the 80m inductivity betwen the inner conductor from
the Matchbox and the inner conductor to the antenna.
With that coil and the Matchbox SWR was almost good, so I added more windings and more and more until
I had 2 ferrit rings full of cable and still the manual Matchbox in the system. I did’t know the $mu_r$
of the ferrit rings but from my calculations I expected something about 90 windings, to much for that
ring. Which after several
days of experimenting, leads to a more radical solution:

image{IMG_20130817_094307.jpg}

Match!!

image{IMG_20130817_101743.jpg}

Because ham radio is still experimental I did not add any shielding to that gigantic coil. 80 windings
on 11cm waste tube, It seems to work, and it is disabled on the higher bands. I surely will soon find
out how bad/good or however this setup performs. 😉