Collinear 8 db VHF Antenna

As some of the club members may have noticed I enjoy making my own antennas and sometimes from the most unusual materials however a quick flick through my log book tells me I must have got something right (well sometimes!).

This project came about as I was looking for a high gain omni directional antenna to use in the field. I needed something that was light weight and could be carried in a small package when collapsed...Oh yes made the most of my 10 watts and didn't rob the bank.

After some unsuccessful experimenting using capacitors for phasing the elements I settled on a stacked j pole consisting of four half wave radiating elements, with each element taken 180 degrees out of phase using a half wave phasing section that separates each radiator. If all that sounds nonsense don't worry I have simplified it by giving all numbers (see diagram at the end of the article) and the materials that I have used but use your imagination if you cannot use or source the same materials.

I started to build the antenna by cutting four lengths of wire, the measurement is 38.75ins for a half wave @ 145mhz but I cut them at 39.75ins to compensate for the joints. The wire I used was multi stranded insulated wire that was from a discarded lawn mower lead.

The end of each length I stripped back ½ in of insulation, the next step I cut 3 lengths of twin and earth and here I used the insulated wire (red and black wires) the length of each wire is 38.75ins again half wave lengths, these are for the phasing sections that separates each element. I folded each in half and mounted them on plastic lids which are used for fast food containers (scrounged from work), using cable ties to keep them secure I soldered the four elements to the three phasing sections. The gap of the phasing elements is 3in...see photos.
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The matching transformer was made from two lengths of alloy tubing spaced 1.8ins apart and screwed to two plastic rails to keep them parallel.
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And the antenna completed ready for taping to a fishing pole.
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This Antenna is DC grounded by the matching transformer and a choke balun is a must to stop the feeder radiating. Here (as close to the feed point as poss) I wound 8 turns of the coax round a piece of waste water pipe.
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The antenna taped to a roach pole which is the biggest out lay and this was purchased from G3CWI (Richard). So yet another use for a SOTA pole!
collinear vhf antenna1I used crocodile clips for sliding up and down the matching transformer, ideally using an antenna analyser while building each section would give max performance however I found if the antenna did not resonate I couldn't achieve an SWR reading below 2:1, with making all measurements exact a reading below 1.5:1 was easily achieved in a couple of minutes.


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