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Aerials.. in depth.
Posted: Fri Apr 28, 2017 6:57 am
by Wolfie G8VAT
Morning fellow ATVers, I have some fundamental problems/questions, I guess general Radio but relevant to ATV Antennae at 23cms, I might simply have even forgotten all this stuff due to years of 2stroke inhalation of course.
I'm looking to optimise my AE system for the up coming activity weekend, I have 3x 23cm Yagi Ants all completely different.. so I can't see any comparisons to be made by measurement, this weekend I'm intending setting something up in the garden to do some actual output power comparisons.
1 was donated and unknown but commercial built, it now has a (home made) dipole instead of a folded dipole so, the directors are all stood off the boom on plastic insulators and insulated from the boom but in the same plane as the dipole, I'd like to work out if it is top, middle or bottom of the 23cm band, I'm struggling to compare to a known design because every one I find online or books is different!
2 is the Severnside 23cm, the Folded Dipole is grounded to the boom at centre point and the directors are in line with this side of the Dipole, each director is screwed through (so connected to) the boom, I can't remember or find if this is top, middle or bottom cos every design online is different.
3 is another commercial design from my own stash and unknown origin (probably a rally purchase years ago). The Folded Dipole is grounded to the boom at the centre point and is extremely close to D1, the directors are on insualtors in line with this side of the Dipole.
SO my questions are.
Using a ruler (or similar) how do I tell what the centre frequency will be?
Which is best, everything insulated from the boom or everything grounded to it?
Which is the best plane for the directors, in line with the open end of a folded dipole, in the centre, or in the plane of the continuous side of the dipole?
Can any of you Antenna gurus make sense of this for me please? 
Muchas gracias.
Re: Aerials.. in depth.
Posted: Fri Apr 28, 2017 8:25 am
by g8gtz
Hi Graham,
You are in danger of starting a religious debate so I will leave those much more capable than me to comment on antennae theory and practice! But a couple of points:
The first antennae sounds like a Tonna - black plastic element stand offs supporting varnished copper wire elements? If so they have a good reputation.
It's very easy to make a basic antennae testing system using your MiniTiouner and the NPM software that comes as standard - it's in the same directory as the normal program. On 23cms you can use your low power ATV or DATV transmitter (or even an ADF4351 Portsdown LO or any other oscillator) in to a dipole at the end of the garden. Set up your test aerial in to your minituner as far away as possible - you won't need a pre-amp but maybe an attenuator on the tx and play. Once you've tweaked the system (not too much / little signal) and played around, you'll be able to get some relative results (not absolute) between your aerials.
See CQ-TV 253 page 27
Watch Dave G8ADM's talk at CAT12 - some very interesting points including a couple of novel feeds for 70cms and 23cms - both of which I use and work really well - plus comments on putting the element through the boom!
https://www.youtube.com/watch?v=ZeNan5_Dbrk
70cms yagi feed described in video is on page 22 of CQ-TV 243
Hope this helps...
73 Noel
Re: Aerials.. in depth.
Posted: Fri Apr 28, 2017 9:26 am
by Wolfie G8VAT
I used to have a Tonna many years ago (wish I'd never sold it but it was HUGE!). Nope, it's nothing like that, they are short plastic mountings that keep the elements just a few mm off the Boom.
I suppose a couple or three pictures would help (when I get home).
That sounds like a similar sort of test setup I was thinking of, just more complicated, I have a satellite signal strength meter, put the RX AE at the far end of the garden and see which AE sends the biggest signal to it, it's gotta be simple for me I'm afraid.
I also want to add a Balun to each one since none already have one, I daresay that's gonna help a lot.
God save us from religion.
Re: Aerials.. in depth.
Posted: Fri Apr 28, 2017 10:39 pm
by Wolfie G8VAT
- 2x Aerials.jpg (103.97 KiB) Viewed 6913 times
The far one is my unknown AE.
The near one is basically a commercial AE but it's had the Radiator and reflector modified (reasons unknown), so how can I check that they are correct?
The severnside is up a pole..
Re: Aerials.. in depth.
Posted: Mon May 01, 2017 5:36 am
by G8DKC
Hi Wolfie,
The closest one looks like a Sandpiper, and with the balun and folded driver works very well.
When I'm back at home QTH will check for the pictures and scribbles made of driven and balun a while back.
Yagi still in use.
73
Peter
Re: Aerials.. in depth.
Posted: Mon May 01, 2017 5:57 pm
by Wolfie G8VAT
I've not heard of a Sandpiper, but I canibalised the far one yesterday and used the Folded Dipole and reflector to rebuild that one, it did improve it no end (and tidied up it's looks too.. They were still loose at this point to allow me to play and find the right spot to drill and secure them.
[attachment=2]IMAG1489s.jpg[/attachment]
It had been the best AE for RX during my testing yesterday, So I have built it as an RX AE for my mobile set up and fitted a Masthead Preamp.
[attachment=1]IMAG1490s.jpg[/attachment]
I made a Balun this morning for the TX AE, everything went well, except, there was less RF output with it fitted... Typical
[attachment=0]IMAG1491s.jpg[/attachment]
It's my first ever attempt at any sort of matching, so I daresay someone might come along and pont out the simple thing I have overlooked that stops it from working as it should.
All my tests have been relative, I either got an improvement or things got worse, so my AEs still might not be brilliant, they are simply not as crappy as they were.
Re: Aerials.. in depth.
Posted: Mon May 01, 2017 7:04 pm
by G8DKC
Hi Graham,
Looks like you are all sorted.
Attached picture of driven element and dims, when bought second hand.
Peter
Re: Aerials.. in depth.
Posted: Mon May 01, 2017 9:52 pm
by Wolfie G8VAT
Thanks for the drawings Peter, I'll leave alone for now as it's a bit close to the activity weekend and I'm back to work tomorrow (for a rest)..
But I'll have another play at a later date..
You are right though, it does look like a Sandpiper, I just found a picture of one on t'interweb, and I must ask Steve why it had been "modified".
Re: Aerials.. in depth.
Posted: Wed May 03, 2017 5:09 am
by G8DKC
Hi Graham,
Now back home so have "paint" for pretty pic of dims
I had to drill holes in the box and face N plug up to stop it filling with water.
Peter
Re: Aerials.. in depth.
Posted: Wed May 03, 2017 12:02 pm
by Wolfie G8VAT
How come there is no direct connection to the outer of the N Type? Although it does appear to be potentially connected to the Boom when assembled.
I had a look at that CAT12 talk too, very interesting apart from people coughing themselves to death into the Mic. He did answer a couple of my very first questions at the top though.
I would like to use a Balun on the TX Severnside, but I'm not too concerned about one for the RX Sandpiper just yet, I expect the difference will not be AS great on an RX AE as a potentially disastrous bad SWR on TX.
But then again, if a Balun makes the Sandpiper better than Severnside at TX their roles may reverse again.. more playtime needed.
Just at the moment though and since my Direct Coax attempt didn't work I probably should convert to Semi Rigid for the last part of the feed but it introduces other problems like secure method of linking, connectors etc. It will have to wait, I have curtailed ATV spending just for a little while.
I did like the look of the centre fed Folded Dipole although his diagrams weren't too clear on what connected where, I daresay that DuckDuckGo will provide some examples.
Thanks for your help again.