We are testing ‘Mamasita’ our new high gain (+7.8dBi) 5/8 co-linear FM Antenna. With 20m of LMR400 it will multiply your transmitter power by 5 times !!!, so with a 200W TX or amp it will radiate 1000W ERP.

I get all the best jobs, I’m the one opening the door. “Careful chaps, it’s taken us three months to get this FM antenna to this stage”

The two main tubes you see the guys holding are aluminium 1 and 1/2 inch in diameter with thick walls, you can see it’s rigid with no sagging even with the weight of the PTFE insulator in the middle.

The design of this FM antenna is strong. The darker coloured tube on the right side of the photo is heavy steel and the bracket on the small square piece is supporting it without any problem.

A nice view of the phasing stub on the far left hand side wrapped around the PTFE insulator. This has now been changed on the next revision, same shape and size, but made of solid aluminium round bar of 1/2 inch diameter. We’ll have pictures of this soon.

This is Elvis (in blue t shirt) and I having a proud moment and looking forward to testing.

Preparing for field testing in an open field right next to our industrial park. It’s just over 30C on the 20th January, beautiful weather. We were trying hard to hold the FM antenna straight and vertical while keeping the ground planes sticking out by the base just over the tailgate of the car. If the tailgate goes above the ground planes then we start to distort the radiation pattern and see reflected power.

You can see a nice view of the tuning bar sticking out of the square piece at the bottom. This is simply a half turn coil or inductor. The tube slides between the two flat bars. At the moment while testing it is held only with two elastic bands, when we have found the correct position, later we’ll drill holes in the bar and secure it in place with a long M6 stainless steel nut and bolt. Also a nice view of the four ground planes. Elvis is moving the bar whilst looking at the SWR meter in the back of the car.

You can see how big this thing really is, Elvis in the photo is about 5 11. On the right hand side picture below, you can see that we have changed the flimsy phasing stub half up the antenna for a strong solid 1/2 inch aluminium round bar formed and bent in to a stub. We are seeing a perfect 1.0 SWR with a bandwidth of +/-3 MHz to the SWR points of 1.5.

Preparing to do some preliminary radiation pattern tests, these are not going to be perfect due to ground being so close, but they will give us an good idea if the main two radiator sections are phased correctly and if we’ve made the phasing stub in the middle correctly.

We are now transmitting just a few milliwatts and I am checking to see if this signal can be received at the end of the antenna, if we see ‘End Fire’ signal here we’ve made a serious mistake with our calculations or construction. So far all’s good, not a single trace of ‘End Fire’ which basically means each of the two main radiators are within the 5/8 of wave limit and the phasing stub in the centre is pretty close to correct.

In the left picture below I am measuring the maximum signal, it’s a little strange as it’s about 15 degrees down on the horizontal. Not quite sure why that is at the moment, I’m guessing the phasing stub is a little off or it’s an effect of the ground, we’ll know soon. In the right picture below I am confirming the antenna is radiating correctly, virtually no signal received again at this antenna end point.

In the left picture below I am starting to see a little signal at this position, the meter is showing about a 30% of the maximum reading. In the right picture below I am seeing 85% of maximum signal.

Here again in the picture below at just 15 degrees below the horizontal we are seeing maximum 100% meter reading. A 15 degree tilt down is a bit of a surprise, not a problem we can’t resolve, but certainly a surprise. Normally with this type of FM antenna we are seeing tilting going upwards when the dimensions are not correct.

By Paul Hollings
https://www.aareff.com/paul-hollings.htm