The BAZOOKA Antenna

[ronald001]

https://www.sirioantenne.it/en/products/vhf/cx-4-m

Can be retuned easily...

[sinus trouble]

A quick update on this project

As the good old British weather has been decent lately, I took this antenna out for a test and the results were not great?

Tests started off promising as i trimmed off small amounts of the top element to achieve a better SWR but i went too far and cocked the whole thing up!

So now i am back to the drawing board in shame!

My thoughts on how to proceed is to get some data instead of random actions!

My plan B is to cut the antenna for 98Mhz (Centre band) then take measurements across the band and plot the results? This should give me some idea of what is actually going on?

I will post any breakthroughs (Or Fails) very soon!

[radium98]

good luck

[mpx]

How did you get on with this Sinus? Did it work or was it a fail?

[Albert H]

I built a couple of "flowerpots" last week. One is for the bottom of the band, and the other is for 98.4MHz. I used the "wrong" aerial connected to the 98.4 MHz rig, and got a 1.6 : 1 match. A but of experimentation with the 98.4 MHz aerial showed that it was <1.65 : 1 across most of the band, with the SWR dropping to 1.01 : 1 at the resonant frequency. I bought plastic pipe and end caps to house them (from Screwfix) and built the whole lot - two aerials, mountings and feeders - for <£30.

[teckniqs]

I built a couple of "flowerpots" last week.

Bill and Ben

[mpx]

How well does the flowerpot work? I'm guessing the pattern and gain is similar to that of a half wave dipole

[sinus trouble]

How did you get on with this Sinus? Did it work or was it a fail?

Well to be honest? I managed to get it working reasonably?

I tried trimming for best SWR but it made no significant improvement.

What i did find is that the SWR was unpredictable when mounted in different locations and very sensitive to any surroundings.

Also i suspect that the coax feedline was also affecting my readings? Which leads me to think some kind of balun is required?

I will do some more testing soon

[MiXiN]

I built a couple of "flowerpots" last week.

Bill and Ben

Flobbadobba flobbadobba

[mpx]

Yeah sounds like the coax is radiating if moving it about and holding it affects the SWR. Maybe a choke balun would do the trick

[teckniqs]

Seen a few people just make a 5 turn loop below their J-poles etc, would that help if directly below the feedpoint?

[mpx]

Or 3 turns on 27" of 7mm coax?
http://ham-radio.com/k6sti/balun.htm

[Albert H]

My "Flowerpots" have 6-turn baluns

[sinus trouble]

I have done a bit more testing on the BAZOOKA and as i mentioned it is an absolute nightmare to get good performance!
Maybe others have had better luck? But for me? Its too unstable for a 'Simple' antenna!

As a Balun would most likely be required? I saved some hassle and went for the 'Flower Pot'

I guess Albert was right........ Again!!

I found a very useful calculator online which also gives the length of the coaxial choke which allows no confusion between number of turns VS form diameter! I will include the link below!

https://nomonsuhendar.blogspot.com/2020 ... lator.html

[sinus trouble]

Here are some pics! Hooray!! It works!!

[sinus trouble]

FP Centre.jpg

[sinus trouble]

FP Choke.jpg

[sinus trouble]

FP SWR.jpg

[Albert H]

There you go - the "Flowerpot" is easy to get going well, and has a good radiation characteristic. I've used a ⅝-wave one on top of a 12m scaffold pole, and it shows an SWR of 1.04 : 1 at its designed frequency. 500 kHz higher or lower, the SWR is still under 1.1 : 1. If you build them for the middle of the band, you could probably use them without tweaking over most of the band! Obviously the best efficiency will be at the designed frequency, but they're useful on any Band II frequency in a pinch.

Incidentally, the 12m scaffold pole is on top of a radiating MW tower!

[mpx]

Presumably for a 5/8 version you just scale up, no other requirements?

Good to know the flowerpot works and the bazooka not so much. Thanks Sinus