MARTIN - G8JNJ

ECLECTIC AETHER - Adventures with Amateur Radio

Omni-directional mixed polarisation antenna for 50MHz & 70MHz



This is a simple 'Halo & Stub' antenna which provides an omni-directional radiation pattern with mixed polarisation, which I built for use with the SUWS WEB SDR 

http://websdr.suws.org.uk/

The design is based on commercial antennas that are used for FM radio broadcasting in the 88-108MHz frequency range. Note that the antenna is quite compact in size when scaled to 50MHz.

The antenna is constructed from standard 15mm & 22mm UK copper waterpipe and fittings, which have been soldered together.

The coax is fed through a 1.4m length of 22mm copper water pipe which forms the boom and which also acts as a 1/4 wave coaxial sleeve Balun. 

The inside of the boom is strengthened with a length of plastic cable conduit which is a tight fit inside the 2mm copper pipe. 

To further improve the antenna balance, I also added three ferrite tubes on the coax inside the boom near the feed point.

The outer of the coax is soldered to the pipe at the mid point of the loop, and the inner of the cable extends out from this point to form a 320mm long gamma match.

Here's a picture taken from the ground below the antenna, showing the various dimensions.





The antenna can be fine tuned by adjusting the thickness of the spacer between the two vertical sections.






Here's another picture of the completed antenna as installed at the Farnham WEB SDR site.





Note that the operational bandwidth of the antenna is quite narrow.





The Horizontal and Vertical gain is quite well matched. Here's a plot taken on an outside test range. Ignore the absolute values shown on the dB axis. 




When using this antenna with JT65 on 6m, it produced about 30% more spots in the same 24 hour time frame, in comparison to a another receiver connected to a 1/4 wave vertical (V-2000) mounted at approximately the same height.





© G8JNJ – 2016




Additional construction notes:-


Dimensions are centre line to centre line.

Constructed from 22mm (boom) and 15mm copper water pipe with Tee adaptor.

 

Plastic conduit inserted in 22mm tube to prevent wall collapsing.

 

Small drain hole in end cap at bottom of lower vertical leg.

 

Drill a hole (7-8mm ?) in the middle of the Tee for the coax to pass through.

 

When coax is pulled through hole, strip back braid for > 300mm and cut braid away about 10mm from coax outer sleeve.

 

Insulate coax outer with heat shrink to prevent melting when heated.

 

Insulate inner near braid with PTFE sleeving or heat shrink

 

Push sleeving back as far as you can under the braid to prevent melting when heated.

 

Fan out braid, pull back coax along boom and place braid over hole in Tee.

 

Solder braid to Tee with blowtorch and quench quickly to prevent coax melting.

 

Add Ferrite sleeve common mode choke on outer of coax inside the boom. Three cores from a PC monitor VGA cable or similar is good.

 

Set the elbow to elbow (a the base of the vertical sections) spacing at about 25mm so that you have room to adjust it either way.

Cut the vertical elements slightly longer than shown so that you can trim to length if you cut the loop slightly too small.

Painting the antenna will lower its resonant frequency, you can compensate by increasing the elbow to elbow spacing.

Wrap two turns of 1.5mm mains cable around 15mm copper tube to form a tap for the gamma match connection, but don't solder it yet.

 

Connect inner of coax to gamma match tap and adjust position of tap and length of gamma match for best SWR (ideally 50 + J0 ).

 

Once best tap position has been found (with antenna raised above ground), disconnect inner of coax, and slip on short length of heat shrink.

 

Solder 1.5mm wire then reconnect inner coax and solder, strengthen solder joint with heat shrink.





70MHz Version