Builders please let me know about your version of the #AirbandBazooka by sending me a message or photos on twitter.
Please do provide some photos/reception reports and I will include them here.
Performance better than most store purchased antennas.
Great feedback received from builders Worldwide. Thanks so much 😀 pic.twitter.com/JidNEAFRub
— merseyradar (@merseyradar) May 24, 2020
A simple enough home brew project for the Airband listener.
Easily the most popular web page on my blog site, Huge Thanks to the many readers world wide , I trust that you will find the information in this article useful to make your own Civil or Military aviation band antenna for very small cost. Made correctly this type of antenna will outperform much more expensive hobbyist grade antenna models available from radio dealers. I hope you enjoy making it,and you are pleased with the performance.
Special Thanks to all my guest readers, some who have taken the trouble to provide detailed construction comments and photos, and to those who have sent messages of thanks to me stating that they are so pleased with the results of their versions. Many times readers comments stated that this page was so helpful in making improvements to their monitoring station and in some cases totally transformed their airband listening experience.
Please do send your build (Please use the comments section at the bottom of the page ) reports/findings/suggestions or photos of your versions. This will help other enthusiasts.
While you are here, have a look around the previous/older posts on my website, you may find other articles interesting.
Antenna Build Notes
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.
I built one for 2 mtrs 144mhz amateur transmissions many years ago and it is still going strong so 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. I was very pleased with the results. Later I moved on to build a version for Military airband. Enjoy the article.
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.
Why Coaxial Dipole ?
Coaxial dipoles are not “super gainers” but they are very broad banded this is particularly useful when designing an antenna that has to cover a wide frequency range such as civil or military airband. Also coaxial dipoles are “quiet” antennas (they are not particulary noisy by design so they are good for hearing weak signals on the band intended) unlike co-linear designs that whilst the gain is good they tend to pick up unwanted frequencies as well as the intended ones resulting in weak signals being lost in the mush.
To explain here is an animation of how the antenna works. As you can see, unlike j-pole,slim jim or co-linear or ground plane antennas it is a totally balanced design.
The current (I) and voltage (V) of the received (or transmitted) signal are an equal value and opposite in polarity along both halves of the dipole.
This is why accurate cutting and measuring are essential, mistakes can cause an imbalance and the antenna will not perform to its full potential.
A very well established and well known Worldwide Commercial RF company Amphenol have 2 products in their superb Procom range of aviation antennas for their Professional/Defence agency customers, Note they have chosen Coaxial Dipole designs… not Co-Linears, not J-Poles or Slim Jims or ground plane designs.
Yes you read right in the Amphenol-Procom specifications…ZERO db gain.
A professional commercial omni-directional and low noise antenna design where reliable coverage in every direction is more important than gain.
Just think of how many radio systems there are at a large airfield..lots , so an antenna design that does not suffer from cross band / intermod problems is used.
Civillian Airband Version
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.
Antenna Elements cable choice
The cable I selected was RG-213, at around 10-11mm overall diameter, it is a much better coaxial to make the antenna elements, RG-213 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, it also has 0.66 velocity factor..
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.
Feeding the antenna..A choice of coaxial..
You can feed the antenna with any 50 ohm or 75 ohm cable of your choice, aircell 7, rg-8x etc are all okay.
I have found over the years that 75 ohm satellite TV coaxial is excellent for feeding airband antennas, in particular Webro WF-100. There are lots of clones out there which are described as “CAI approved CT100” and similar names but no manufacturer is given in the sales listing…keep away from Philex and Labgear they are not anything like Webro for quality. The original and best in my opinion is Webro, the branding is laser printed on the sheath, Heres the spec sheet which says it all..forget rg58 for vhf/uhf its rubbish compared to this. Take note of the attenuation figures for the airband frequencies we will be listening to now compare those with say RG-58MIL or RG-MINI8 specifications. Why use a below par cable ? Lets get all that signal back to the radio where it belongs !
You can see that the braid is dense and tightly packed, also there is a copper foil screen to stop unwanted signal transfer from house wiring etc.
This Webro cable has a Velocity Factor of 0.81, very good indeed for feeding the antenna but a poor choice for making elements for the antenna itself, The reason being you require a inefficient “leaky” coax element of 0.66 Velocity Factor to absorb as many signals over as much of the desired band at the antenna, then transfer the signal down a very efficient coaxial to the radio.
To get anywhere near the performance figures for WF-100, your choices are limited..aircell 7 or Times Microwave LMR-240 perhaps, But..the trouble is ..those choices are more than 4x the price and not much better performance wise.
You can buy 50mtrs of original webro WF100 for around £25
Heavy Duty F-Connectors with o-ring seal, easy, strip (properly without damaging braid) & screw-on to fit.
And then you may need F female adaptors F-SMA, F-BNC, F-PL259 to suit your antenna and radio systems.
Stripping Webro WF100 can be challenging if you use the wrong approach.
By far the best way is to invest in a coaxial stripper, there is a purpose made tool by Dow Corning Cabelcon of Denmark, this comes in a kit with 20 F Type connectors.
The tool works really well and as shown it preserves the foil around the dielectric foam. After stripping The copper foil and braid is undamaged. You could not get a finish like this with a knife.
The tool has 2 spring loaded blades which cut into the coaxial perfectly, it also has an 11mm open ended spanner at the other end for tightening connectors.
The type of F connector shown here is a Cabelcon CX3 Self–Install™ variant. When the body of the connector is pushed on to the coaxial, it has a self locking mechanism keeping the connector in place. I have tugged at them a lot and never managed to get one off without mechanical means or remove one in tact for that matter. Usually the coaxial will fail before the connector grip/lock.
Here is a link to the catalogue of the Cabelcon range of connectors.
Cabelcon of Denmark are a large connector manufacturer, they have been around for ages. Their F Type connectors are second to none quality wise and they are not that expensive..Blue band connectors suit WF100 or RG6.
You can get the connectors (x20 pcs) and the WF100/RG6 stripper for £18 GBP from Amazon UK (Seller is based in Germany), a bargain.
Here is a short video I made showing the coaxial stripper in action.
Tip : When the Cabelcon “quick install” F connector is mounted onto the cable as shown above for the first time use you may experience that the connector threaded shell will not mate too well with the Female F and the threads will not screw up too well. This is because the inside of the connector barrel (sitting level with the dielecric of the coax) is slightly convex, this is intended design by the factory you will need the supplied 9mm spanner tool to tighten the connector up for the first time, this flattens out that convex area. You will only need to do this once per connector.
Tighten up the connector with the spanner then loosen it again, from then on you can easily hand tighten the connector.
If you need a twin coaxial to feed 2 antennas, Webro WF-65 is worth looking at, the specs are much the same as WF100 but WF-65 is a shotgun profile twin 5mm diameter coaxials. This cable does not like sharp bends..gradual bends are okay though. WF-65 takes smaller mini 4.5/5.0mm F connectors.The inner core is quite fragile and this cable needs to be stripped and terminated with care. This twin coaxial is used for B-Sky B /Sky HD tv installs.
Best performance is achieved if you run the feeder coax 90 degrees to the antenna for about 3-4 feet at the feed point.
Making the Antenna
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.
Some planning needs to take place in the first instance.
Decide if you want a loft mounted antenna suspended from a short cord (use a 2 way conduit box as shown) or a mast mounted antenna (3 way conduit box or conduit tee is needed for this version). Then decide if you want to install a connector on to the box as shown in both examples below or solder the coaxial feed line directly to the antenna elements which simplifies things a bit but makes ease of future maintenance a little less easy. All possibilities are right and will work fine, you have to decide and plan which variant will best suit your station.
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 !
Strip back 6mm or so of the opaque dielectric material until you reach the copper centre core, Take care with the antenna element section from now on as the ends could snap off (if handled roughly) until the junction is supported later on by heat shrink tube.
Solder the wire to the braid and centre conductor making a direct short circuit, wrap the wire around the bare copper centre core then solder (see diagram).
Antenna Ends Wire Link Details
Apply a length (50mm long) of heat shrink tubing over the joint and then apply another slightly larger diameter length (75mm long) over the top of that layer to provide strength (For details see Military Airband Version later in this page). Alternatively you could use self amalgamating tape. The antennas strength will return and now you do not have to be so careful when handling.
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 parallel condutor speaker wire has an impedance of around 72 -100 ohms give or take.
Some people have connected with 2 tails made with the stripped back braid, all good except when it comes to removing the so-239 or troubleshooting/ investigating issues.
Removing the so-239 from the braid tails will be challenging…If you are doing it this way ,take care to ensure no tiny strands of braid stray away from the bind and short the antenna at the wrong point.
Some builders solder the feedline direct to the antenna elements..again all good ,You save the cost of a connector.. Antenna will work fine but depending on the coaxial feedline run servicing/maintenance of the antenna in the future may be difficult.
Alternatively if you do not have speaker wire/rig power lead you can use single wires.
If you are going to do this plat the wires as this will make an interface to the antenna of very close to 75 ohms impedance, In longer lengths of feedline twisting the wires helps to prevent interference. This method has been used for decades, Radio Amateurs in the 1940s used to prefer twisted lighting cord as an antenna feed line a very long time before coaxial cables were invented. Old technology/Theory ? No, I think not… Take a look at the inside of todays cat 5 or cat 6 ethernet cables, or any regular telecoms cable or data cable the twist method is used for each pair to prevent breakthrough from unwanted signals/chaff. Sometimes “Old School ” is good !
A photo sent in by reader and airband bazooka builder “EV” from Malta who is using this method to make connection to a DIY Civil Airband Bazooka Antenna.
This will make a nice termination to the antenna, solder each end of the speaker wire or the pair of single wires to the braid exposed making sure they do not touch each other.
Before soldering up the centre junction, ensure the wires are laid in the direction towards the centre of the antenna keeping them as close as possible not away from it to ensure a low impedance (70-100ohm) connection,( parallel twin speaker wire or twisted single wires are good because the distance between each wire is very regular keeping impedance low) laying them the wrong way may create an unnecessary high impedance point (300+ ohms) feeder transition . hard to explain in writing so here is a diagram.
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.
Here are some results by Builder Tommy @tomppa77 (Twitter)
And the results from Tommys nano VNA, Very Good !
UPDATED AUGUST 2019 (Following many requests ..Addition of Details for the Military Airband Version)
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 Follows.
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.
A diagram of the layout for the Military Airband Version Centred at 300mhz.
Before starting, A few words of caution..as with the Civil Airband version it is very important to be very accurate with your measurements , poor measuring and/or cutting will very likely impair performance. Ideally the antenna should be exactly the same dimensions both sides of the centre connection.
Here is the cable cut and stripped ready to make the antenna. At the exact centre of the dipole , around 12-15mm of the outer sheath and braid have been removed and two small notches have been cut in the sheath at each side of the exact centre (exposing the braid slightly) for soldering to the SO-239 later. Two more notches have been cut in the sheath @ 8.49cm from each end to solder the wire shorting links.
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 you build both antennas Civil and Military dont be surprised if the civil one outperforms the dedicated mil air version in the area of 255mhz up to around 270 mhz, at this frequency the civil bazooka behaves like a full wave folded dipole, the Military one is a half wave or there abouts.
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