Bi-quad quandary....

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radiogareth
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Bi-quad quandary....

Post by radiogareth » Fri Oct 15, 2021 11:53 am

I've a need for some decent gain for /P on 24cms and also something for my local repeater, so I built a bi-quad using one of the many on-line calculators. Previous ones I've built for 3.4GHz seem to match up as expected with my NanoVNAV2.
I built one as per the directions, making a precision 3-D printed bending jig to help and 2mm diameter ECC for the element. However, it really does not look like its a good match and changing the distance from the reflector makes no real difference.
Here's a plot from my EU1KY antenna analyser, the NanoVNA largely agrees.
Only thing it does NOT have ATM is any sort of choke at the feed-point?
Pointers very welcome....
Sides are 60mm, target freq 1250.
24CMQUAD.PNG
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Bi-quad.jpg
Bi-quad.jpg (207.77 KiB) Viewed 4606 times
Thanks
Gareth

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PA3CRX
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Re: Bi-quad quandary....

Post by PA3CRX » Fri Oct 15, 2021 2:00 pm

Hi Gareth,
Looking at the photo of your antenna it looks obvious to me that the coax is not connected the way it should.
The shielding of the coax seems to be removed between the reflector plate and the bi-quad element?
The shielding should not be removed, feedpoint one side to the centre pin and the other to the shielding (and do not solder it to the reflector plate).
(By the way, your construction makes depth adjusting difficult, more easy if you use some kind of plastic clamping feedthrough, leave the coax intact).

If you want to do it completely correct (going from balanced to unbalanced) you could add a quarter wave wire in parallel with the coax, one side connected to the pin and the other to the shielding of the coax (therefore an additional hole needs to be made in the plate). Without this additional wire you should also be able to get a proper SWR.
Just google on 'Pawsey Balun'.

Cheers,
Chris PA3CRX

radiogareth
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Re: Bi-quad quandary....

Post by radiogareth » Fri Oct 15, 2021 2:23 pm

Thanks Chris, I'll implement those suggestions and report back :-)
Not sure quite how I missed that (other than by copying the web page https://buildyourownantenna.blogspot.co ... lator.html) as I didn't when I built the little 3.4GHz dish feeder - picture below...
Every day is a school day (and sometime again it seems ;-))
3.4BiQuad.jpg
3.4BiQuad.jpg (70.37 KiB) Viewed 4578 times
Thanks
Gareth

radiogareth
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Re: Bi-quad quandary....

Post by radiogareth » Fri Oct 15, 2021 2:56 pm

Well that was 'easy' - just got to design some smart 3-D printed supports for it now. This is without any balun and with the coax outer isolated from the backplane.
Gareth
24CMBIQD.PNG
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PA3CRX
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Re: Bi-quad quandary....

Post by PA3CRX » Fri Oct 15, 2021 8:24 pm

Good to hear this positive news!

I looked at the website you mentions and indeed not in accordance with the way it should be.

The one I made (for 24 cm) was measured in the field (with distance of the element to the plate 47 mm for best SWR):
gain: 7.8 dBd, -3 dB point left: 43 degrees, -3 dB point right: 63 degrees. Front to back 21 dB. Likely with the Pawsey Balun the radiation direction more will be more symmetrical.

Chris PA3CRX
Last edited by PA3CRX on Sat Apr 02, 2022 7:09 am, edited 1 time in total.

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keng8vdp
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Re: Bi-quad quandary....

Post by keng8vdp » Fri Apr 01, 2022 7:01 pm

CQTV 180 & 182 mag 1999 describe something similar.
I made one all those years back , but re-scaled for 2.4ghz. after we more or less abandoned 13cm FM TV due to interference from emerging wifi
etc it languished in my coal-place for more than 20 years.

Then came QO-100 and a remembered it. Dug it out burried beneath lawnmowers , old tyres, etc.

Currently i'm using it on the NB transponder. Yes its 3db lost as its horizontal. Im S9 in with 10 watts.
S5 using a 3w (8w Chinese) wifi amp.
I can move it OFF alignment by 20 deg either way and still get in !
Vertical alignment has to be spot on.
Its pointing directly at the satellite.
You can make out the 4 way power splitter on the rear behind the chicken wire reflector 32cm square.
On the front are 4 sets of loops sitting around 1" in front of the reflector.
The loops are made from the single strand copper earth from the inside of mains cable.
The off-stands are plastic PCB type with a hole drilled through for the wire supports.
Dabs of epoxy hold things in place.
The feeds from the power splitter are solid semi-rigid.

Because its only just over a foot square this might be attractive to all those that have the desire to go /p

Tested works good also on the Tuesday Night 13cm contest SSB using Langstone/Pluto
Anyone wants a sched on QO-100 to work me using this old manky thing on the uplink .. send me a message ( QRZ.com for email )
Attachments
2.4ghz grid antenna.JPG
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IMG_20200826_WA0013_jpeg.jpg (232.23 KiB) Viewed 3499 times

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PA3CRX
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Re: Bi-quad quandary....

Post by PA3CRX » Sat Apr 02, 2022 7:12 am

For some dimensions if you want to make a version for 24 cm:
https://pi6ats.nl/doppelacht.htm
(not for the stacking part)

g0mjw
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Re: Bi-quad quandary....

Post by g0mjw » Sat Apr 02, 2022 8:59 am

I was thinking of building a similar antenna for use up in Bolton for 23/24cm for a repeater input. The requirement is a wall mounted, almost flat panel with extremely broad 180 degree South azimuth coverage and narrow elevation, so it doesn't need rotating. There is no requirement to beam north as it would have to go through the house. I could put another on the north side, but the remaining 800ft bulk of Winter Hill within a mile is so good at preventing signals getting there that it is probably not worth it.

In practice I think such an aerial needs to be an array of stacked dipoles or perhaps a pair of stacked double quads or patch antennas. It's hard to get more than 90 degrees, without making it into a phased array with adjustable phase, so easier to build more panels and select the best direction with a dual RX. Hence I was thinking of making each panel of two or three stacked bi-quads. The challenge will be in combining them without the combiner loss exceeding the combining gain.

Mike

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PA3CRX
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Re: Bi-quad quandary....

Post by PA3CRX » Sun Apr 03, 2022 8:29 am

Since the azimuth angle will not even be close to your needs I do not think this is a real solution for you.
If you could mount the panel antenna with a small distance of the wall, with a kind of pivot, you could may be make two or three ‘positions’. Depending if you are able to reach the panel possibly by hand, or with some other technical solution.
Thinking about something with an electro magnet or some rope-loop and a small motor, or even a motor of a (car) window sweeper.

Just thoughts,
Chris PA3CRX

Just think out of the box, as I did to make a rotor from some scrap:
https://youtu.be/fqMsIqD6jRc
and
https://youtu.be/03D3JBTwBj0

g0mjw
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Re: Bi-quad quandary....

Post by g0mjw » Sun Apr 03, 2022 9:11 am

Hi Chris

Maybe I wasn't clear. Cover all that Azimuth range simultaneously, hence needing two or probably 3 panels, no moving parts. I have a beam for directivity.

Mike

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