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 Long wire - strange swr results 
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Post Long wire - strange swr results
Hi again - before starting on an inverted L antenna I thought I would throw up some wire to get started quickly. So 66 feet of wire, roughly 20 feet above ground, connected to a 9:1 unun. Trim till good swr. Well that's what I thought. I couldn't get the swr under 10 according to my SARK100 analyser. I'd cut off a fair few feet of wire by this time ( roughly 4 inches at a time) so thought I'd start again.
This time I didn't cut any wire to start with.
I decided to check the coax cable and analyser. I connected the analyser to my OCFD and saw less that 2:1 accross all the 20mtr band. Dummy load showed 1:1 so analyser looks ok. Took the coax feed off the OCFD and connected it to the 9:1 unun.

Checked 20mtrs - still greater than 10:1. I put a gwhip 1:1 current balun in line still the same. At the unun I disconnected the earth wire that was going to my earth spike in the garden. SWR now 2.56 at 14.49MHz. For some of the tests I was using 1KHz step size so with the very small bandwidth I'm getting there could have been a match somewhere but I skipped over it.

Summary table

Freq ------- SWR ----- 1:1 Balun ---- Earth
14.49 ____ 2.56 ____ Yes ______ No
All 20Mtr __ >10 ____ Yes ______ Yes
All 20Mtr __ >10 ____ No ______ Yes
All 20Mtr __ >10 ____ No______ No

Now although I got an swr of 2.56 it was only below 4 from 14.485 to 14.495
So 4 questions
1) Why do I need the 1:1 as well as the 9:1 ?
2) Why does the 9:1 need to have the earth disconnected (I've checked a dozen times - I don't have the radiator and earth mixed up)
3) Why only 10Khz bandwidth?
4) Could the fact that there's a BBC LW/MW transmitter pumping out 50Kw/100Kw less that a mile from me have anything to do with it? (but why doesn't it affect the OCFD?)

Thanks - Colin MM5AGM


Sun Dec 11, 2016 8:27 pm
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 Long wire - strange swr results 
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Post Re: Long wire - strange swr results
Whats the reading at 40? As your description sound like you have an end fed halfwave

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Sun Dec 11, 2016 9:25 pm
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 Long wire - strange swr results 
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Post Re: Long wire - strange swr results
OK I will have a go but I simply don't have the knowledge or experience of other amatuers on this forum. In my opinion I also think you have an end fed half wave antenna there for 40 metres

I have read the web site of Fabio Bonucci IK0IXI (who might be known to other QRP enthusiasts on this forum that attended the Gqrp convention in Halifax back in 2014) . Fabio is a well regarded home constructor, QRP operator and CW enthusiast

Fabio has experimented with 'end fed' wires including a design that he himself calls the "Olandesina"
he has an excellent web site but unfortunately the information is not in English

However his 'multiband' version of your antenna features a 9:1 unun mounted at height fed with RG58U cable with an RF choke at the feed point of the unun consisting of ten turns wrapped around a 30cm former. There is one counterpoise wire from the unun 10 metres long that is left dangling down vertically and then the rest trailing on the ground . The drawings are quite good so you should be able to work it out from that

http://nuke.ik0ixi.it/Antenne/EndFed/ta ... fault.aspx

I hope that helps in some way keep experimenting ;) kind regards Adrian

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Sun Dec 11, 2016 9:32 pm
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 Long wire - strange swr results 
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Post Re: Long wire - strange swr results
If you want to use a long wire on 40m and 20m, then there is one length to avoid ... and that is 66 feet.

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Sun Dec 11, 2016 9:42 pm
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 Long wire - strange swr results 
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:) sorry forgot to say don't use a resonant wire with a 9:1 unun . Around 16 metres of wire should work better .... :blush: er at least I have found with my own meagre experiments

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Sun Dec 11, 2016 9:48 pm
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 Long wire - strange swr results 
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Post Re: Long wire - strange swr results
G6CSL wrote:
If you want to use a long wire on 40m and 20m, then there is one length to avoid ... and that is 66 feet.


Doh - didn't think enough before picking length to start with.
Thanks again - I'll chop a length off and try again.
Colin MM5AGM


Sun Dec 11, 2016 10:20 pm
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 Long wire - strange swr results 
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Post Re: Long wire - strange swr results
As others have said, your 66ft wire is about a half-wave long on 20m. Its impedance will be very high - typically several thousand Ohms - let's say 4500.

Your 9:1 Unun probably wont give you a 9:1 impedance transformation when handling such an extreme load, but let's say it did - that would reduce the antenna 4500 Ohms to 500 Ohms at the coax connection ...... which is an SWR of 10:1! You shouldn't expect anything better.

Removing the earth from the Unun produces a very odd arrangement where the Unun winding is now in series with antenna; almost any results are then possible.

Steve G3TXQ

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Sun Dec 11, 2016 10:26 pm
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 Long wire - strange swr results 
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M0RDI wrote:
:) sorry forgot to say don't use a resonant wire with a 9:1 unun . Around 16 metres of wire should work better .... :blush: er at least I have found with my own meagre experiments


As I get older I get more forgetful. I should have checked the 66 foot bit for resonance or maybe I need to start working in metres (66 feet = 20.12 metres). Pretty obvious now!!
Thanks again
Colin MM5AGM


Sun Dec 11, 2016 10:27 pm
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 Long wire - strange swr results 
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G3TXQ wrote:
As others have said, your 66ft wire is about a half-wave long on 20m. Its impedance will be very high - typically several thousand Ohms - let's say 4500.

Your 9:1 Unun probably wont give you a 9:1 impedance transformation when handling such an extreme load, but let's say it did - that would reduce the antenna 4500 Ohms to 500 Ohms at the coax connection ...... which is an SWR of 10:1! You shouldn't expect anything better.

Removing the earth from the Unun produces a very odd arrangement where the Unun winding is now in series with antenna; almost any results are then possible.

Steve G3TXQ


Thanks Steve. A senior moment on my part. I'm going to start working in metres from now on. It's pretty obvious when you do that, 66 feet = 20.12 metres.
Thanks again - Colin MM5AGM


Sun Dec 11, 2016 10:33 pm
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 Long wire - strange swr results 
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Post Re: Long wire - strange swr results
mm5agm wrote:
M0RDI wrote:
:) sorry forgot to say don't use a resonant wire with a 9:1 unun . Around 16 metres of wire should work better .... :blush: er at least I have found with my own meagre experiments


As I get older I get more forgetful. I should have checked the 66 foot bit for resonance or maybe I need to start working in metres (66 feet = 20.12 metres). Pretty obvious now!!
Thanks again
Colin MM5AGM


:) Your welcome Colin and please take a look at Fabios Web site. It's a wealth of information :wave:

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Sun Dec 11, 2016 10:42 pm
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 Long wire - strange swr results 
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Post Re: Long wire - strange swr results
mm5agm wrote:
4) Could the fact that there's a BBC LW/MW transmitter pumping out 50Kw/100Kw less that a mile from me have anything to do with it? (but why doesn't it affect the OCFD?)

Thanks - Colin MM5AGM




Yes, it is possible, but could easily be checked with an ordinary SWR meter.

Some analysers warn that damage could occur with a 100W tranmitter with 100m, so it is a possibility though unlikely.

I get better results with a 5:1 unun


Mon Dec 12, 2016 3:53 am
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 Long wire - strange swr results 
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Post Re: Long wire - strange swr results
I have the same analyzer and can confirm it gives 'strange' readings if there's a nearby big signal - in my case a 800 Watt broadcast station on 999kHz about 1/2 mile away which was inducing a couple of mW's of power into my antenna.

I fitted a 1.5MHz 7 pole high pass filter ahead of the Sark and problem solved, at least in my case.

73 Tim

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Mon Dec 12, 2016 10:00 am
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 Long wire - strange swr results 
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G4WIM wrote:
I have the same analyzer and can confirm it gives 'strange' readings if there's a nearby big signal - in my case a 800 Watt broadcast station on 999kHz about 1/2 mile away which was inducing a couple of mW's of power into my antenna.

I fitted a 1.5MHz 7 pole high pass filter ahead of the Sark and problem solved, at least in my case.

73 Tim


Yes - BBC at Westerglen is causing the analyser grief.
I started again with 60 foot of wire and started chopping bits off. SWR never came below 10 and I got down past the 59 foot length witch should have been not too bad. Thank you Bob MM0RMR for the length and swr chart. I then put together a high pass filter with 1 cap (470pf) and 1 resistor (220 ohms) and immediately saw the swr at around 3.5 on 15MHz. I chopped another bit of wire off and could see that the resonant frequency had changed. I reckon Westerglen is still swamping the analyser so will need to build/buy a better high pass filter. At least I feel I'm getting somewhere now.
Colin MM5AGM


Mon Dec 12, 2016 7:19 pm
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 Long wire - strange swr results 
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mm5agm wrote:
G4WIM wrote:
I have the same analyzer and can confirm it gives 'strange' readings if there's a nearby big signal - in my case a 800 Watt broadcast station on 999kHz about 1/2 mile away which was inducing a couple of mW's of power into my antenna.

I fitted a 1.5MHz 7 pole high pass filter ahead of the Sark and problem solved, at least in my case.

73 Tim


Problem now solved. As per Tims, G4WIM, suggestion, I also fitted a 7 pole high pass filter but designed for 2.5 MHz ahead of the Sark and problem solved. Everything now works as expected.

Thanks for all the help.
Colin MM5AGM


Sat Dec 17, 2016 9:13 pm
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 Long wire - strange swr results 
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Post Re: Long wire - strange swr results
Glad you got it sorted and everything makes sense.
Another lesson learned.

73 Tim

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Sun Dec 18, 2016 9:09 am
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