I decided to make a simple coaxial dipole for civil airband monitoring, easy to make and widebanded. Provides very good performance throughout the 118-136 mhz range.
The antenna is known as a “double bazooka” design and is very popular with radio amateurs who make it for 144-146mhz FM operations.
Here we will adjust the lengths from that basic design so that the centre frequency is around 128mhz which is right in the middle of the civil airband.
First of all the plan .
The calculations were done using an online program “coaxdpl.exe” by VE3SQB which is a very handy program, it does all the hard work of calculating the correct element lengths in relation to changing velocity factors of many common coaxial cables and it is available as a small download from this page.
The cable I selected was RG-213, at around 10-11mm overall diameter, it makes a wider banded antenna than thinner rg8x or rg-58 types.
RG-8 cable is also good for making these antennas, its more or less the same as RG-213 ,for our purposes anyway..
The most important thing is to select a cable with a 0.66 velocity factor for making this antenna.
The dielectric of 0.66 vf cable will be a cloudy/milky transparent type, not a solid white like foam.
You can feed the antenna with any 50 ohm or 75 ohm cable of your choice, aircell 7, rg-8x etc are all good.
Best performance is achieved if you run the feeder coax 90 degrees to the antenna for about 3-4 feet at the feed point.
Now to getting on and making.
I used a 20mm conduit box and some 20mm electrical conduit tube from my local screwfix hardware store the lot cost about £4.50. The RG-213 I bought an odd end of 2 metres from a local dealer for £1.50.
Now to drill the hole to accept so-239, a 16mm bit will suffice.
Now to drill the securing holes for the so-239, A 3mm bit is ideal for this.
After offering up the SO-239 and marking off, drill the holes.
You should end up with something like this.
Now lets deal with the coaxial cable which will form our antenna elements.
Heres the layout and the measurements, forgive the clothes drying prop but this made it much easier measuring the cable out !
I used 16mm wide electrical tape to mark out the cuts 58.5cm to the centre and 20cm from each end whilst the cable was flat and straight against the prop support.
Now lets get on with stripping the cable as shown in diagram 1.
Heres the exact centre of the antenna, an area at 58.5mm distance from each end.
Strip back around 15-20mm of the exact centre section leaving the centre conductor in tact, be careful measuring, remove the outer sheath and braid.
As seen I have used a piece of 16mm width white electrical tape as a guide, the exact centre of the tape being 58.5cm from each end of the coaxial.
Here, Measure 3 times to be really sure, and apply tape once.
You will have something that looks like this, make sure that the stripped back section is equal distance from each end of the antenna.
Check twice, 3 times perhaps, this is important.
Now strip back the 2 ends, again be really sure, measure 3 times, apply tape marker once.
You should have something like this
Now make the join at the ends with some loose wire, I chose 26awg PTFE /teflon silver coated aircraft harness wire, a little part of a 100 yard consignment I bought from a salvage yard in Arizona, USA.. at stupidly cheap money I might add.. wonderful cable , extracted from the avionics bay of a DC-10-30 I understand…very apt !
The dealer gave me the reg of the donor aircraft ! I will keep that to myself !
Solder the wire to the braid and centre conductor making a direct short circuit.
Wrap the joint with some self-amalgamating tape.
Now we return to the centre to finish off.
Mark out a hole in the centre of the rubber gasket.
Cut out using a small screwdriver as a dibbler .
Now the SO-239 , A piece of speaker wire has an impedance of around 72 -100 ohms give or take.
This will make a nice termination to the antenna, solder each end of the speaker wire to the braid exposed making sure they do not touch each other.
Heres the equipment ready to terminate.
Joining it to the coaxial, 2 lengths of black PVC tubing tube were cut so that ,allowing for the box in the centre, the overall length of the antenna and termination box when assembled is around 1.2mtrs.
Lay the coaxial inside the box and now work at the centre junction to make the connections.
Solder them up making sure there are no unintended short circuits. Note the small cable tie to secure and the m3 stainless bolts have been fitted to the SO-239.
To make a join to the SO-239 outer shell I reamed out a 3mm hole in a flat spade crimp terminal, then bent it over 90 degrees back on itself and soldered it on to one end of the speaker wire to make a neat termination.
Keeping the speaker wire tails as short as possible but not so much as to interfere with soldering the centre.
Fit the lid to the box and you should have something like this.
Now fit the 2 lengths of tube cut earlier making the antenna total length around 1.2 mtrs , fit some 20mm end caps (bought from ebay £2 for 20 pcs) and you are all done.
Hang up in loft from either end with some string.
The antenna is a great all rounder and perfect for loft mounting.
Best of all you can make it yourself very cheaply and stand back and admire your work, making antennas and experimenting is fun.
Military Airband Version.
You can make one for military airband with centre freq of 300mhz using the same method but here I have chosen more popular, widely available RG-8 cable (again this has 0.66 velocity factor), use these plans but obviously you would mount it vertically polarised for am airband transmissions.
The whole antenna measures less than 0.5mtrs end to end.
You can feed the antenna with a 50 or 75 ohm cable of your choice.
300mhz Military Airband Version Detail.
20mm black condit 3mtr length around £2.00 GBP
20mm black through BESA box with gasket, around £1.20 GBP each .
SO-239 chassis socket £1.50 GBP each.
RG-213 or RG-8 around £1.25 per metre. ( hint : 2 mtrs makes a civil and a military version)
M3 machine screws and nylock nuts, around £1.00 GBP pack of 20 pcs.
20mm black pvc tube bungs from ebay, about £2.00 GBP for 20 pcs.