We have continued to play around with the Coax cable vertical antenna and I think it is sorted. I still need to do a lot more on air tests to make sure it does work significantly better than the Wilson 5000 but we seem to have tamed the RF currents that were on the braid and causing the SWR to rise as power was increased. It took about 10 turns around a toroid choke (ferrite FT240-43) placed 1/4 wave in free space below the Coax coils. The SWR stayed constant as we increased the power from 4 watts, to 10 watts and then 50 watts on AM. No problem for the RG 58/C cable I have used. I only had a 40 ch radio to check the bandwidth Channel one was 1.2 and Channel 40 was 1.4 so we could fine tune things further but I will sit on this for a while. Looks like we should get a MHz under 1.5:1 which would suit me fine. Easiest way to fine tune this antenna is just to wind the air coil up or down an inch or two. I don’t want to cut the radiator just yet. Covering the NZ truckies on 26.450 AM can’t be a bad thing. Most active part of the band around these parts it seems.
The picture below is basically what we built. An end fed 1/2 wave antenna I am told. Works like a dipole for gain and take off angles which is not that bad. I saw on the net that Sirio also market a similar antenna in a fibreglass stick called the gainmaster HW.
The picture in the manual sure looks the same! I think mine is easier to carry around as long as you have either a fibreglass pole to stick it up in the air or something to hang it off. Sirrio probably use better coax as well…hope so ..to justify the cost. And someone in Germany has also come out with something that sounds the same but without any real pictures and dimensions one can’t be sure. There is one thing for sure: It ain’t magic. Its easy to build yourself. Just one bit of soldering; the plug on the end of the coax.
The only crucial part is the coil. It seems to depend more on the length of coax cable in the air coil than the number of turns or the diameter. 5 or 6 close spaced wraps around 5 or 6 inch diameter will get you close. The rest just falls into place. I started with 2.65m of radiator and the same from where the braid is cut to the top of the coil. First tune by moving the coil up or down and find how it is changing the SWR. Follow it to get around 1.2 to 1 on the frequency of choice. The SWR was fine on 5 w carrier but rose as the power was increased. If I added or removed wraps on the air coil the SWR increased so I added a toroid choke 1/4 wave in air below the coil and ‘voila’ flat SWR under power!!
It fits my needs for an easy to deploy vertical antenna. The 30′ pole stays up with out guying in a moderate breeze. It can take a few hundred watts with ease. Now I just need to prove it radiates significantly better than the Wilson. One thing for sure. When the wind is blowing, its cold and it is pissing down with rain I will be snug in the car with the wilson no matter what!
Initial transmit testing does not show much difference with signals of 5 or more on the Wilson. On the receive I am seeing 2 to 3 S points on the meter. The big difference on ground wave is when the signals are weak or low. Then it does lift the signals out of the noise. It should do. It is about 6m higher in the air so hardly a fair test! Have not had any chance to test it with Skip as the skip has been dead of late.
The Coax ‘Cobra’ laying out on the ground
For those interested here is what I did to build this antenna.
Parts: 15m of RG58/C coaxial cable. (Non foam, Non aluminium shield), One PL259 Plug, tape measure, yellow insulating tape, scissors.
Work out the dimensions from the formula 300/fx (mhz) x .94 for wavelength of the wire.
((300/27.3) x.94)/4 for a quarter wave. For 27.3mhz it came to 2.582, which I rounded up to 2.65 meters to give me some wriggle room.
Measure down 2.65m and place a wrap of yellow tape under the 2.65 mark. (Tape 1)The top of the tape should be 2.65. Cut off the black polythene jacket to expose the braid and remove the black plastic without messing up the braid underneath. You will end up with 2.65m of braid showing.
Pull the braid down and over the black plastic outer. Bunch it up and it slides over OK.
When it is all pulled down the center conductor in its solid plastic jacket will be exposed. All 2.65m of it. Now carefully cut the braid at the yellow mark. Use sharp pointed scissors. You don’t want to snip or cut into the plastic inner or it will weaken the antenna.
Now you will have the braid over the coax and free to move. Slide it down towards the bottom and tape it in place temporarily so it doesn’t move around. We can use the braid as a sleeve later on so best leave it on the coax.
Next. Measure 2.65m down from the top of the yellow tape under the white conductor you have just exposed and wrap a turn of yellow tape. (call this tape 2) This will be the top of the turns of the air coil. For no good reason other than to complicate matters I decided I would use an electrical quarter wave of coax in the coil. I used 2.65 x 0.66 (the velocity factor of the this coax is around 66%. That gave me 1.75m.
Measure down 1.75m from Tape 2 and wrap another yellow tape around this mark (Tape 3) So now we have 3 wraps of yellow tape on our coax. The first one is under the exposed inner conductor, the second is 2.65m down from that on the black shield and the third is 1.75 meters down from there or 4.40m down tape 1.
Coil up the 1.75m of coax below tape 2 around a 6 or 7 inch former. Has to be neat and in line. No cross overs.
The braid. After soldering the plug to the coax I stretched the braid down the outer and soldered it to the outside or ground of plug. I think this should probably be 1/4 wave in air but the braid wasn’t long enough so I just used it as it was. End up with about 2 meters or so. It made a difference at lower power levels so I left it on. (Lowered the SWR) If it works for you cover it in heatshrink. It only connects at the plug.
That’s about it. If you are going to use 100w or so you could add the toroid as pictured above and can probably dispense with the braid. I placed the toroid a quarter wave or 2.65m under the coil and wound it guanella style, in one side and out the other.
I just tape the antenna to the pole every 75 cm or so as I push it up to prevent it flapping about. The antenna radiating part is everything above the coil. (Fingers crossed!)
How to tune.
3 things can be altered to affect the tune. The length of the radiator (exposed white inner core) and the length of the black outer above the air coil. (and the coil itself) First work out where it is at resonance..Should be resonating lower in frequency using these figures and wind the coil up to shorten the bottom section. 1 or 2 inches at a time to start with to see which way the SWR is changing. You can add to the coil or subtract a turn as well to get a feel for things. The swr will change as you change the height above ground so try to take the swr readings at the final height if possible. Last thing is to cut or bend the top radiator to bring the SWR down to around 1.2 or so across the 40ch. Actually anything 1.5 : 1 across the band is good.
Basically : if your dimensions are in the ballpark you just need to play around with the coil to change the swr. It is quite reactive and any metal objects nearby can cause the swr to change. Just rearrange coil to suit!
Appreciate any feed back.