The 1/4wave
Groundplane Antenna is an easy type to build and requires few and easy to
find materials.
Basically it is a copper wire/rod whip attached on a SO-239
chassis connector with 4 more pieces of wire radiating (radials) from the
center to create what is called artificial ground (plane)
Our Version
In our case we maintained the principle, but we departed on the way parts
are connected. We always try making our antennas packable, so we moved forward
with making all elements detachable.
An added bonus is that the detachable elements allow the center whip to
be replaceable with others, pre-tuned to different frequencies, thus allowing us
to cover more regions in the VHF band.
Materials Used
1 chassis SO-239 connector
2 meters of 3mm thick single strand copper wire
1 bronze soldering rod 3mm thick
1 electrical wire screw connector
Several barrel-type terminal connectors
Making the
Antenna
The basic idea is using various kinds of electrical connectors that are
easy to procure and solder on.
Below is what the locally found connectors look like and the way they were
attached to the radials.
Radials were made out of stretched 3mm single stand electrical wire, cut
at 50cm long.
Detailed view of how the
connectors were disassembled and prepared to be soldered
Then 4 grooves were made on the chassis connector body (2) with a round file, and the
radials’ sockets were soldered (2).
The connector center stub was shortened and an electrical wire screw connector
core was attached with solder (1).
Then we attached the whip on the UHF connector.
The whip is a 3mm brass rod, the kind that is used for brazing. We initially
cut it at 49cm but tuning the antenna brought the length lower.
The finished and assembled antenna with 2 of the radials removed.
Advantages
Well the obvious one is that the antenna can be disassembled for easy storage
and carry.
Also by exchanging whips it can be used for various frequencies. In practical
terms one can have several pre-tuned whips, use them as radials, and just chose
the proper one for the desired transmit frequency.
Disadvantages
The 1/4wave Groundplane Antenna shows practically no gain,
offering an almost spherical radiation pattern. That can be of concern but if
range is what you are looking for sometimes pattern shape is of more
importance.
Thoughts
While working the whip attachment method can be improved.
If one is to hoist the antenna on a field expedient mast that is of a
concern but hanging the antenna form a tree takes any load of the whip and
makes for a more reliable operation.
Furthering the design could take the way of having a whip socket that
would allow moving the whip up and down as the mobile antennas do.
Tuning and
Performance
Preliminary reading by a SWR meter were unexpectedly good.
We measured 1.01 all throu 144.150 to 145.850 MHz, the Greek HAM band being 144.000 to 146.000 MHz.
Respectively SWR at 140.000 is 1.37 and at 150.000 it is 1.43!
And that was all. Now we are waiting for a chance to field test the design.
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