There are plenty of amateur tactical aerials available on the market for 2m and 70cm but I couldn’t find an airband one. I decided to cook one up, but couldn’t find any recipes. I wanted an end-fed half-wave (EFHW) tuned for 125.000 to 127.000 Mhz. We use many flower pots aerials, rubber duckies and quarter waves aerials, none of which were practical for the field table portable setup. After many researches, ideas, help from fellow amateurs and many attempts, I ended up with a suitable antenna.
The measuring tape attachment is made of a piece of aluminum shaped to the measuring tape pieces – face to face. Brass would be better, I didn’t have any. It could be 3D printed, including the inductor former as part of it. The total length of the radiating element from the top of the measuring tapes to the top of the inductor is 1.18 m. The feedline is 3m of good quality RG58, longer the better for EFHW.
The circuit diagram is as follows:
For the inductor former I used part of an 8mm plastic coat hanger …. oops … don’t tell anyone.
The coil comprises 11 turns of 1mm enamel copper wire, with a 1mm gap between each turn. The RG58 braid is connected to the bottom of the inductor. The radiating element is connected to the top of the inductor.
70 mm of RG316 is used as the matching capacitor and is connected to the center wire of the RG58 and tap at 4.5 turns from the bottom. To get the best match, I started with 100mm of RG316 and cut until it was no longer improving, if you cut it too short, replace the wire and start again!
Here is the result on the antenna analyser, looks pretty good for the target frequency range!
But now, time to insert the set-up into the PVC sleeve … a bit of squeezing and pulling … well the chart hasn’t improved but still within the tolerance! Lesson’s learned, perhaps next time I’ll split the PVC pipe and sandwich the set-up so it can be installed in a controlled manner.
Next step will be the heatshrink.
Field test!
Stay tuned – pun intended! That thing could be a sophisticated dummy load! I’m lucky to be at a location where I have access to 3 airport ATIS (Air Traffic Information Systems): a strong one (YSSY – Sydney Airport), a weak one (YSBK – Bankstown Airport) and a very weak one (YSRI – RAAF Base Richmond), all in the frequency range I’m targeting the aerial for. I’ll give it a go and report back.
Test results (26-Jul-2023)
From three receiving locations I’m getting 1 to 3 S point better signal with the tape measure aerial compared with the rubber ducky when the top of both aerials are at the same height. An EFHW aerial is often referred to as a ground independent antenna, however many articles explain this is not the case and not providing adequate grounding or counterpoise may be the source of many issues. What I noticed with this project is that I get a near perfect analyser reading when the feedline is laid down on the ground, but a not very good analyser report when the feedline runs on the table. I can also improve the frequency broadness when stepping on the feedline. Unsure how well these aerials perform when connected directly on a handheld with no counterpoise. A coaxial dipole (Flower Pot Aerial) is far less sensitive to the environment and would be my first choice when its use if practical.
How was the matching circuit calculated?
Got few requests on how I came up to this circuit, so I better explain. I’ve watched many “tear down” videos on half wave mobile antennas and tried to clone the circuits with very little success. I thought it would be so simple if only I had a manual VHF antenna tuner, tune and read the values and recreate the circuit with components …. BING!!! … I decided to build an antenna tuner at the bottom of the antenna that would become the circuit when the match was found. If you look closely at the test bed picture you’ll notice a wire to tap the inductor from the bottom of the antenna.
As for the feed line, I used a 2-8 pF trim pot. All “tear down” video I watched seemed to be taping at half turns, so started at 2.5 and trimmed to best SWR and resonance. Then moved to 3.5, etc. Once I found an acceptable match, I replaced the trim pot by 100mm of RG316 and trimmed to match, knowing that somewhere between 100 and 10 mm I’d find my match.
Reverse engineering
Here is our attempt in reverse engineering the circuit using SimNEC. C2 and C3 represent stray capacitance, C1 is the capacitance of 71mm RG316 (RG316 is 29pF/ft or 6.6pF for 70mm). The auto-transformer was calculated by deriving the inductance of 11t of 1.1mm wire with a coil length of 22mm (i.e. single-spaced winding) at 0.47uH (or 470nH). The feedline length has little impact on the Smith Chart.
No Comments Yet