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Which Match?

S

Simon004

Member
Aug 16, 2009
82
2
18
Lancashire, UK
I have used a homemade 2 ele quad for a few years now, fed with a commercial 1:1 balun and a 1/4 wave of 75 ohm coax, which has worked very well. I wanted a little more gain, so last week I extended the boom and added a dipole for a director, so now I have a Quagi (cute name huh). Problem being now I have a 2:1 swr, a friend of mine suggested a clemens match, which I built, but found it gave me a very narrow bandwidth, in the region of 200kHz between 1.5:1 swr points, which is not good enough (for me anyway). Maybe I did something wrong or didn't build the match correctly, I'm not sure. I could just make a gamma match, but I'd prefer a balanced matching circuit. A T-match is supposed to be very hard to tune, so not really keen on that either.

Looking around the net I've stumbled accross the hairpin match, seeing as I have a 1:1 balun already on the antenna, this seems like the ideal solution, although I've also read that this match will reduce the gain of the antenna (allthough only by a few tenths of a dB). I've also read that to use the hairpin I would have to reduce the length of the driven element by 3%, which I'm not keen on doing incase I ever want to go back to using the antenna as a 2 ele quad. The main problem is all the infomation I can find is all relating to a yagi antenna, and my driven element is a quad loop, so I'm not 100% sure that the calculations/measurements given for a yagi will work for me.

My question is, all these different matching systems seem to have different pro's and con's, but from your experience (that I know you guys have a lot of) which would be the best match to use on my antenna? "Best" would mean: cleanest/most balanced radiation pattern, lowest loss in the matching circuit, and also ease of construction.

Remember my driven element is a quad loop, supported by a non-conductive spreader, and I already have the 1:1 balun on there. I thought I'd ask you guys before I take the plunge and buy the materials to make the hairpin match.

Thanks in advance...
 

What is a "clemens match"?
Typically, adding elements to a quad, such as a director, will lower the input impedance of the driven element. Just for grins, remove that 75 ohm matching section and see what that does to the SWR.
- 'Doc

Adding elements to a multi-element antenna of any sort will typically lower the input impedance. That's not just for quads.
 
Rob,
That 75 ohm coax was used as a matching section, which is fairly common. A typical 2 element quad probably has an input impedance of around 100 ohms, give or take a few. A 1/4 wave of 75 ohm coax would get that down much closer to 50 ohms. Adding another element to that quad means the input impedance will drop to something around 75(?) ohms, sort of (no idea what it will actually be since it depends on a few other factors). No matching at all would yield something like a 1.5:1 SWR, give or take a bit? That's better than 2:1.
- 'Doc
 
Doc,

I should have been a bit clearer, after adding the director, with the 75 ohm transformer swr was 5:1, trimming away at that 75 ohm coax only reduced swr to 3:1, removing the 75 ohm completely and feeding directly with 50 ohm coax gives me the 2:1 swr. As I can't tune the antenna with the 75 ohm coax I'm guessing I have an impedance of around 25 ohms at the antenna, which is why I now need some sort of gamma match to raise the impedance to match to the 50 ohm feed.

The Clemens match is here : Clemens Match, It did work, but the bandwidth was very narrow.
 
Well, my opinion sort of matches yours, probably the input impedance is around 25 ohms give or take some. If you could find some 35 ohm coax, I'd say try an electrical 1/4 wave of that and see what happens. Having doubts of there even being any such thing as 35 ohm coax, that's sort of a useless idea, I think. Then again, what about paralleling a couple of those 75 ohm matching sections for a resulting impedance around 35 ohms? Think that may work? Got a bunch of that 75 ohm coax you don't mind wasting? Try it! I expect a commission if it works. If it doesn't work, it was all your idea, not mine! (Hows that for a CYA??)
Honestly have no idea if it would work or not, just a S.W.A.G.
- 'Doc
 
As Doc says if your antenna does have a 25ohm input impedance the obvious solution is paralleling 2 x 1/4 waves of 75 ohm cable,(remembering too figure in the velocity factor),ideally rg11/u.

there is two ways you can do it,either solder both the cables inner conductors and shields together at both ends which leaves the job of waterproofing it and difficulty fitting connectors,or ideally you could fit pl plugs on all the 1/4 wave ends then use two y splitters one at each end to join them,ideally one splitter would be 2xso239 and 1xpl259 to go into the balun which i'm assuming has an so239 socket,and at the other end you would want a y splitter with 3 x so239 to accept the 50 ohm cables pl259 and both the 1/4 wave matching systems pl 259's.

the main trouble you have is you are trying to tune an antenna with a swr meter alone,so you can't be sure the impedance is 25 ohms,it could just as equally be 100 ohms or for that matter some other value plus lots of capacitive/inductive reactance.an antenna analyser would be much better tool to see exactly what is happening although isn't always affordable.

I'd say Docs solution would be a good starting point without the availability of an analyser.
 
Cheers George/Doc,

I don't have access to an analyser, so there is indeed a lot of guess work going on here. I don't have masses of 75 ohm coax, I do have some good quality RG59, but only a couple of metres of it, I'm sure I have some cheap TV coax I can experiment with to see if it works, and then upgrade it to lower loss stuff later on. I actually saw the paralelled 75 ohm technique in the ARRL antenna book, but using lots of splitters and plugs doesn't really appeal to me, it's worth a shot though.

I have to say I'm suprised that the impedance is 25 ohms. As a 2 ele quad it was around 100 ohms because I matched it with 1/4 wave of 75 ohm coax. So from 100 ohms to 25 is a big drop for just adding 1 more element right? I think I'll try a 75 matching section again and cut it down a little slower.
 
Interesting info, fellows. The summer being an otherwise busy time for me, I've been too busy to play with antennas. Hopefully, I'll be back to it soon and this should help me as well.
I have used the doubled 75ohm matching section in another experimental system before with decent results, so thanks for the reminder.
 

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