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 and much larger versions hung horizontally for HF operations.
DOUBLE –BAZOOKA ~ COAXIAL DIPOLE BROAD-BAND DIPOLE
Originally developed by the staff at M.I.T. ( Massachusetts Institute of Technology) for radar use, and featured later in QST Magazine published by the ARRL (July 1968).This antenna consists of a half-wavelength section of coax line with the shield opened at the centre and feed line attached to the open ends of the shield. The outside conductor of the coax acts a half-wave dipole in combination with the open wire end sections of the antenna. The inside sections, do not radiate, but act as quarter-wave shorted stubs which present high resistive impedance to the feed point at resonance and tends to cancel reactance as frequence off resonant frequencies, thus increasing band width.
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.
The image above shows my original plan for the outdoor version which I built some time ago, unfortunately I did not take any photos of the construction.
I thought about making another one for loft mounting which is much the same idea but the horizontal boom is omitted as this would make the antenna cumbersome to move around in the confines of an attic or loft space. The loft version is easy to hang up in to the roof timbers with string and a hook, run the feeder cable at 90 degrees for at least 58 cm for best results.
A note in the above image regarding the coaxial feedline. I use Webro WF100 75 ohm Satellite TV coaxial cable for all my listening antennas, its relatively inexpensive and has excellent performance right up to microwave bands.
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.
UPDATED AUGUST 2019
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.
Military Airband Version construction notes (August 2019)
I was asked to make a loft mounted military airband version for a friend so here are the details.
Here I am using RG-213 coax, and centre frequency 300 mhz, the calculator gives the same results as for RG-8 shown above.
Here is the cable cut and stripped ready to make the antenna.
I threaded some heat shrink tubing on to the coaxial before soldering up all the joints, then I used a hairdrier to shrink the tube down, this gives some strength to the end sections and provides some mechanical strength to the centre solder joints. For the centre, I used some 7/0.2mm PTFE coated wire and kept these as short as possible. I soldered a ring terminal for the grounding point on the SO-239.
A close up of the centre connection detail, after a little heat from the hairdrier, the tubing fits nicely.
The SO-239 with some more smaller heat shrink tubing fitted and an M3 nyloc nut to secure the ring terminal to shield side of the connector.
Note the double layer of heat shrink tubing (black over yellow) applied to each end.
The antenna is nearly ready, just some PVC tube 22cm to cut for each end, about 20mm sits inside the PVC tube socket on the centre box itself so that around 20cm/200mm is visible each side when the antenna is completed.
Now a photo of the completed antenna.
Quick accessory/ addition, I drilled a 6mm hole in the centre of a spare end stop / bung and made a parachute cord loop for hanging on a hook or screw in the loft space. The end user can swap out one of the blank end stops for this mounting option.
A finishing touch from the label printer.
If you want to be fancy..Once you have made the tube cuts and have everything looking good, glue the tube to the centre box using PVC solvent based cement.
This usually comes in a metal can with an applicator brush attached to its lid.
Follow makers instructions carefully…do not be over enthusiastic with this stuff…by design the product melts the tube surface so apply only to the parts of the tube that are not seen as they sit inside the sleeves of the centre box..about 15mm each side of the centre terminal box.
For readers in the UK here are 2 examples one on the left from Screwfix, the one on the right from Toolstation..
One thing though, This stuff is great and will seal everything really nicely..especially for outdoor version built with 3 way box and horizontal boom…so good in fact it will be almost impossible to remove the tubes with out mechanical means once the “cold weld” has cured fully but please make sure that no solvent adhesive gets inside the tube and on to the pvc coaxial outer sheath of the elements and, of course,as with all solvents keep this away from the usuals..Flames, Kids/Pets.
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.
PVC solvent cement if you want to be really fancy..costs about £3-£6 a tin…but that will do a lot of antennas !
UPDATE MAY 2019:
If mounting externally, It is a good idea to drill a drain hole into the bottom bung and leave free air to circulate inside the tube, I used a 10mm hole but I didnt include the details in the write up , thanks to Adam from Australia for pointing this out.
With the drain hole, there will be no chance of condensation or icing.
You may decide that 10mm is too large and you may want to try 3-6mm or so, big enough for the ambient air to circulate and for water to drain.
You may be in a cold location where ice is an issue, in this case a bigger hole would be more difficult to ice over ,seal up and cause problems.
Much depends on your own location and weather conditions.
Where this antenna design really excels is the very low noise floor which enables you to hear stations which are normally unreadable/ buried in the mush when using a co-linear or similar.
If you build one of these, use the comments box below to let me know how the build went for you and more importantly the performance of this antenna against your normal station rx antenna.
In May 2019, I received correspondence from Adam VK3SWL in Melbourne Australia.
He had seen this article and decided to construct the outdoor version using 3 pieces of tube as shown at the top of this page but interestingly he made some adjustments to my original plan. Adam came up with some small design changes which I think are significant and well thought out so I asked him to write about his findings and contribute to this page.
As you can see from his photos in the section below,Adam has a nice little antenna farm going on over there in Melbourne, over to you VK3SWL for your report..
Report by builder Adam (VK3SWL).
“One thing I wish I had done was taken photos of the construction process.
Didn’t think to do this, since it was a spur of the moment build upon realising I had the parts in stock and the time to give it a crack.
The build pretty much follows how you have done yours, but with a few differences:
I used a 3 way electrical junction box in the centre, so I could make mast mounting easier.
The PVC conduit is 25mm grey, so a little larger diameter (would have loved black like yours but cannot find any locally).
As previously mentioned, I used some bubble wrap around the co-ax element ends to keep it from rattling in the pipe.
Instead of drilling the junction box lid for the SO-239 socket, I drilled the rear of the junction box and mounted the socket there keeping the lead lengths to the co-axial elements as short as possible. Actually the braid of one element is soldered straight to the ground lug on the SO-239 and a small piece of RG-213 inner core (1 strand) was used to connect the other element to the centre pole of the SO-239.
Then the whole inner connections of the antenna were flooded (potted?) with hot melt glue to minimise corrosion and movement.
Once happy with the pipe lengths over the elements, I glued the pipe into the centre junction box.
Also drilled a 3mm hole in the lower end cap, since I had noticed condensation in there when I was testing the antenna on a portable mast for about a week.
The end caps have also been glued on with hot melt to seal it all up nicely and keep out the spiders.
A 1 metre piece of 25mm aluminium tube is used to mast mount the antenna, as using PVC might have resulted in too much flexing and bouncing of the antenna, since we do get westerly winds up to 80km/h (sometimes 100km/h) being a geographically flat part of Melbourne (basalt plains). The co-ax is cable tied along the aluminium tube to the mast. Also glued an end cap on the aluminium mount tube too.
The connection to the SO-239 socket has been bound in 3M self-amalgamating rubber tape and covered over in electrical tape.
I later thought that it might have been a good idea to mount the SO-239 inside the junction box and have the coax run inside the aluminium tube to keep the connection out of the weather.
I have mounted the dipole up as high as can be done without it encroaching on the Diamond X-6000 vertical above it.
It is around 6 metres of the ground and offset just under 1 metre from the mast.
While the mast was down (it is a tilt over mast which bolts to the side of the house brick wall) I decided to replace the ageing RG-58 that was feeding an old VHF folded dipole.
The folded dipole was tuned for 148-170Mhz so not really much good on airband, so down it came.
Coaxial cable is now LL-195, which is the same physical size as RG-58, but is semi rigid, double shielded and has a solid inner conductor.
Much less loss than RG-58 and has a better outer sheath material which will withstand the searing Australian sun better.
The dipole has been up on the mast for a little over a week now, and is performing very well.
There is now a good couple of S points difference between the dipole and the discone with the dipole outperforming the discone.
I am using an SSE JIM airband filter between the radio and antenna which helps with noise when the bands are busy.
My location is 22Km south west of the city of Melbourne, with Tullamarine (Melbourne) airport about 35Km to my north.
The airport is slightly lower in elevation compared to me (14m HASL) so it is difficult to receive ground traffic.
I can just hear ground traffic now on the dipole, compared to just noise on the discone.
Receivers in use here are an AOR AR5000 +3, Icom IC-R7100 and IC-R8500, which can be fed to the dipole via a Stridsberg MCA-204M multicoupler.
I am still in the process of comparing results to the Diamond DJ-130, but can say without doubt, the dipole is a much better performer!
A really worthwhile project, that can be built for very little (if any) money and performs very well!
My intention now is to build another one for field use and maybe one cut for the 166Mhz PMR band.
Please see attached pictures!
Note the drain hole in the bottom.
Thanks again for a great blog site!
Adam VK3SWL. Melboune, Australia.”
Thanks Adam, some nice ideas there.
I had not thought about drilling the rear of the box for the SO-239, the PVC is much thicker and stronger there.
Using a shorter link between the SO-239 and elements, flooding the junction from the lid side with hot melt glue and the drilling of a drain hole were good modifications.
I drilled a larger 10mm hole but in Australia I dont suppose you have to worry about ice build up..
So using this build , Adam found 2 or 3 S-points improvement over his much more expensive Diamond D-130 discone which is mounted slightly higher on the mast.
Nice results and thanks for the report.
Dont want to make your own airband antenna ?
Check out these options from UK based Superyagi antennas