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 Help Needed with Bow-Tie Beam 
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Post Help Needed with Bow-Tie Beam
I'm a little puzzled with what I am experiencing with a newly bought antenna so could do with some help.

I purchased a Bi-Quad or Bow-Tie beam for 70cm, (whatever you prefer to call it before Xmas), it took a while to arrive because the manufacturer said their analyzer had failed and they would only ship the antenna after testing. It's basically a bow-tie antenna with director elements in front of the loop as well as the usual reflector.

Anyway I got the antenna up on a pole last weekend in time for the 70cm UKAC contest on Tuesday, now conditions were difficult so it's probably not a fair comparison but I expected much better results based on the manufacturers claimed performance figures.

So I've had chance to play a little today and get the antenna analyzer on it, now I don't have sofisticaed equipment, I have the Times Technology T100 VHF/UHF antenna analyzer.

The antenna has about 2.5m of URM67 coax to a pre-amp and then a long run of Westflex to the radio. I picked up the URM67 from Maplins last week as I didn't have time to get 213 from a different retailer.

With the pre-amp in line I get the following results:
432Mhz SWR is 1.59, R is 35 and X is -12
432.25Mhz SWR is 1.61, R is 35 and X is -13
423.5Mhz SWR is 1.67, R is 34 and X is -13
432.75Mhz SWR is 1.71, R is 32 and X is -13
433Mhz SWR is 1.76, R is 31 and X is -13

If I take the pre-amp out of line and join the coax's with an inline connector I get the following results:
432Mhz SWR is 2.12, R is 33 and X is -27
432.25Mhz SWR is 2.24, R is 30 and X is -26
432.5Mhz SWR is 2.32, R is 28 and X is -24
432.75Mhz SWR is 2.36, R is 26 and X is -21
433Mhz SWR is 2.35, R is 25 and X is -18

If I go to the antenna and connect the analyzer to the 2.5m length of coax, removing the long coax run the results are silmilar to above but SWR is lightly higher.

I then removed the 2.5m of URM67 and conected a 2m long piece of mini-8 that I had hanging around and the result was quite different, I didn't plot all the options as the weather is crappy but at 432.25Mhz the SWR dropped to approx 1.4, R was 45 and X was +18

Steve, can you help me make sense of all this?

On the surface it would appear the piece of URM67 is causing an issue although the results with the mini-8 aren't great either but I need to get my installation issues out of the way before I blame the antenna or start looking to fine tune it's resonance.

Thanks

Richard


Sat Jan 15, 2011 3:03 pm
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
That's not an antenna I'm familiar with.
Can you post a link?
Have you tried checking the antenna with a short length of Westflex?
How long is the main run?
Have you tried terminating the cables in a dummy load and doing checks on that?
At least you would know about the cable then.

Conditions on Tuesday were not brilliant but there was a reasonable level of activity.

I've never used cable from Maplins but, in my experience, when all the suspect cable was about it was always marked as 213.

Cheers

Tony

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Sun Jan 16, 2011 10:48 am
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
Richard,

Obviously the most meaningful measurements are those made through the shortest feedlines - the ones with 2.5m of URM67 and with 2m of mini-8.

Just the 0.5m difference in length could produce very different complex impedances, so the change in values of X between the two cables doesn't necessarily tells us anything. However, the big improvement in SWR is significant and would seem to indicate a problem with the URM67 cable/connectors.

How does the SWR vary across the band using just the 2m of mini-8?

Steve G3TXQ


Sun Jan 16, 2011 12:33 pm
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
Thanks for the replies Tony and Steve, I'll do some more testing during the week and post the results for your comments. I picked up some 213 yesterday so I'll make up another feeder approx 2.5m and see how it compares.

Thanks

Richard


Sun Jan 16, 2011 8:52 pm
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
Okay finally got to do some more testing this afternoon:

Did some tests with a dummy load direct to the analizer and then at the end of the coax, I checked the mini 8, the section of UR67 and a section of 213 that was on another antenna but actually happened to be almost exactly the same length as the UR67 just 10mm difference. I tested at 432Mhz and also at 145Mhz, the resistance of the dummy load connected direct on the analyzer dropped to around 40ohms at 432Mhz and was around 54ohms at 145Mhz

The dummy load at the end of the mini 8 consistantly measured a higher resistance than the dummy load directly on the analyzer.

The dummy load at the end of both the UR67 and 213 consistantly measured lower resistance than when connected on the analyzer.

I think that explains why I saw such a difference when I previously switched between the UR67 and mini 8 when testing the antenna.

The results for the UR67 and 213 were very similar, between 1 and 3ohms difference so at this point I assumed there was no coax issue.

I then connected it all back up to the antenna for one final test and search for resonance.

The closest point to resonance was found at 446Mhz. Now I'm starting to wonder if the manufacturer has sent me the wrong antenna and this one was built for PMR use instead of 70cm?

Anyway guess I'll be on to them next week for a replacement or my money back, this is actually the most expensive antenna I have ever purchased at 200 Euros and clearly the least successful.

I have no idea how to model this but if you are up for a challenge Steve I would get some measurements for you!

Richard


Sat Jan 22, 2011 3:49 pm
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
Hi Richard,

M0WBN wrote:
The dummy load at the end of the mini 8 consistantly measured a higher resistance than the dummy load directly on the analyzer.

Don't get too concerned by those results - small changes in length make a big difference at these frequencies. For example a change in length of just 11cm could transform an impedance of 40Ī© to 62Ī© at 432MHz.

M0WBN wrote:
I have no idea how to model this but if you are up for a challenge Steve I would get some measurements for you!

I'm always up for a modelling challenge, but I don't know this antenna so you'd have to sketch it out or point me at a picture. You'd also have to measure dimensions very accurately - it would only take a 1cm error on a dipole to move the tuning from 432MHz to 446MHz.

73,
Steve G3TXQ


Sat Jan 22, 2011 4:21 pm
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
Steve,

If you can send me your email on a PM then I will scan the information that came with the antenna and send it to you, then you can decide if it's worth trying to model.

Richard.


Sat Jan 22, 2011 4:30 pm
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
Richard,

eMail address at the bottom of this page:
http://www.karinya.net/g3txq/

Cheers,
Steve


Sat Jan 22, 2011 4:43 pm
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
Steve,

Sent you an email, let me know if you got it and the attachments came through, had an issue with my normal email so used my yahoo account instead and I'm not sure if it will come through with the large attached file.

Richard


Sat Jan 22, 2011 8:04 pm
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
Richard,

Received OK, thanks.

I didn't realise it had that many elements! If I decide to give it a go I'll pm you for the dimensions; wouldn't want you to go to a lot of trouble and me not then model it.

One thought: I haven't checked the maths, but I wonder if a quick measurement of one element - for example Director 1 - would be enough to tell whether you'd been given one tuned to the wrong frequency?

73,
Steve G3TXQ


Sat Jan 22, 2011 8:37 pm
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
Yeah, it's got few hasn't it, sorry about that.

If it helps the 1st director is 265mm total, the remaining directors are approx 274-275mm.

The 1st director is 42mm infront of the loop and the loop is 160mm infront of the reflectors.

The reflectors are 380mm long and the additional element marked as 435 resonator on the drawing is 275mm long and 50mm behind the loop.

Let me know if that's enoug for a basic assessment.

Thanks

Richard


Sat Jan 22, 2011 11:00 pm
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
Do you have a link to this antenna as I'm not sure it's one I've come across before. :?

It should not be for PMR446 as external antennas for that band are not legal.

What make is it, I wonder?

Cheers

Tony

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Sun Jan 23, 2011 12:19 am
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
The suspense is killing you isn't it Tony?

Sorry to keep you waiting, all will be revealed if I can get it working right, I even missed you in the last 70cm contest and you're normally a definite for the log.

Richard


Sun Jan 23, 2011 12:46 am
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
Richard,

I'd hoped the dimensions might give a clear clue that you'd been supplied with an incorrect version - but I don't think they do. The Director lengths look shorter than I would have expected, however the Reflector lengths are longer.

The data sheet you sent for the 70cm version clearly shows the maximum width as 38cm, and that seems consistent with what you are measuring.

Not sure that helps a lot, but it does argue against it being a wrongly supplied part, particularly if as Tony says there would be no market for a 446MHz version.

I confess I don't know what the 435MHz "compensator" does; does it look like it might be a tuning adjustment?

73,
Steve G3TXQ


Sun Jan 23, 2011 10:11 am
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 Help Needed with Bow-Tie Beam 
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Post Re: Help Needed with Bow-Tie Beam
Steve,

Maybe I'll just play a little today while I wait for an answer from the manufacturer next week.

The only parts that can move are the parasitic elements that can slide from side to side within the clamps so that isn't going to do anything for resonance, I could try sliding the 435 resonator to one side but I assume that would effect the radiation pattern.

If I can find some 5mm rod in B&Q I might try a slightly longer 1st director and just see if it has any effect. The spacing is all drilled through the boom so I don't want to mess with that just yet.

Thanks

Richard


Sun Jan 23, 2011 10:48 am
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