Transmission1

The following is just for fun really, during "lock down".

I have been experimenting (aka playing) with one of these (using it vertically) =



I have put LOADS of different mobile aerial combinations on it, including a full size 1/4 wave (both as radiator and counterpoise).
Even 2 x 2 foot "Dial-a-Match" aerials work and I did get contacts from 20 miles away even with them !!
The results in terms of SWR have been generally quite good.
I have not at this stage been noting down "real world" performance (yet).

There is of course a commercially made equivalent of this, known generally as a Boomerang =



My question is, on those, the counterpoise is always shown at a distinct angle to the radiator.
Why is this?

I know nothing about the antenna you are talking about, but one effect of angling the radiators will be to alter the feed impedance, maybe it was a crude/simple way of matching to 50 ohms?

A half wave centre fed dipole antenna in free space has an impedance of 73 Ohm.
When angle between bottom part and radiating element is 135° it have 50 Ohm.
Chris

As you say a 135 angle gives 50 ohms.

Local objects will alter it some but the adjustment on the boomerang allows testing. Years ago I found these -3dB down on compared to a full size half wave. Not a bad compromise for local operation.

ch25 wrote: ↑19 Apr 2020, 12:34 A half wave centre fed dipole antenna in free space has an impedance of 73 Ohm.
When angle between bottom part and radiating element is 135° it have 50 Ohm.
Chris

As I have seen so often Chris, your answer is good and straight to the point

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I added a "DV banjo" to to that "straight" dipole centre connector (pictured above) so I could experiment with angles. I can now confirm that putting the counterpoise at an angle does indeed reduce the SWR compared to having the elements inline

It currently has a full size 1/4 wave on top and a loaded 1/4 wave underneath and tunes very well. Initial tests indicate it is working reasonably well too.

However, the joke is that, in view of all this, I have now ordered a proper (full size) Boomerang
The reason being that the dipole centre I have been playing with does not look very strong (this was also mentioned elsewhere here).

It's been summat interesting to do though

Wow, I never knew that about the angle altering the impedance! You learn something new every day

How about a CB aerial resembling this 1950s TV aerial:
http://www.wrightsaerials.tv/aerialphot ... /048.shtml
It could be a period accessory for buildings in conservation areas

Aye, that TV aerial is a strange one Chris

Things have moved on. I bought the Sirio Boomerang (the full size version) and it worked fine on top of my 4 Mtr telescopic fibre glass pole.

I went to a lot of trouble getting an adaptor made so that I could use std 3/8" thread mobile aerials as the vertical element. That proved fun and helped pass a day or two of lockdown, experimenting.

However, I looked at it and thought the mounting system did not really suit my application and I thought "I can do neater than that".

The result was this arrangement, which fits the pole perfectly and is neat, compact and versatile (Note = The pole is not fully extended in the pictures.)

That looks well engineered
Tell the posh neighbours it's the new Brabantia heated outdoor ecological clothes dryer

ChrisMagpie wrote: ↑28 May 2020, 08:22 That looks well engineered

Nice of you to say that, but really all I did was a bit of lateral thinking, gather together the ready made bits n pieces and then drill one 3/8" hole in that mirror mount and assemble it

It has been fun testing combinations of different "twigs" top and bottom. Even 2 x 2 foot long "Dial-A-Match" ones tuned and worked (though not sure about how well they worked distance wise )

Two 2' Dial-A-Match angled to resemble that old TV aerial, and fitted to the apex of the building, might pass as permitted development :)

Once the negative leg of an antenna like yours is angled it is more akin to a 1/4 wave GPA (ground plane antenna) and yes the angle alters the
feed impedance. It may be ever so slightly directional with just one ground plane. I think officially a GPA needs 2 x ground plane radials to be classed as such. The GPA is an excellent antenna when there is space available for the 3 or 4 ground plane radials.

I would love to use one but impractical for my typical garden tree / mobile static setups.