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Τετάρτη, 22 Οκτωβρίου 2014

A different take on the VHF 1/4wave Groundplane Antenna


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)

For more information the antenna is detailed in this article A Quick and Simple 2 Meter Ground Plane Project

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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