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Old 29th Sep 2019, 3:40 pm   #1
M3VUV51
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Default Dipole for 10 metres issue.

hi all,im trying to string a dipole up in the loft for 28.300mhz,ive made both legs at 2.52 meters long,which should be in the ball park,its made of pvc coated speaker wire,on im using a vk5jst analyser,that says its 1.3 to 1 swr,however my swr meter says its over 10-1 swr,also my rigs atu wont match it either,its clear of any metalwork and wireing by about 8ft,any ideas why this is.ps.all the coax checks ok,thats rg-213about 12ft long,cheers m3vuv.
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Old 29th Sep 2019, 5:13 pm   #2
James Duncan
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Default Re: dipole for 10mtrs issue

Ten meter band, a few inches makes a difference,
Reduce your TX power to about 10 watts,no tuner in use! find at what frequency you get the lowest SWR, You must remember that the dipole has an impedance of about 73 ohms and your coax is 50 ohm so the best real SWR one can get is 73/50 about 1.5-1.00 anything below 2-1 is good.
If you get less then something is wrong.Coax length, the 12 ft is near to three quarter waves,you could try odd numbers, ie try 15, 17 or 19 ft
If the best SWR frequency you find is higher than the desired frequency then lengthen the legs a bit, if lower shorten the legs, a few inches make a difference
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Old 29th Sep 2019, 6:06 pm   #3
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Default Re: dipole for 10mtrs issue

Hi,

Just an unlikely thought, but you haven't connected your SWR meter back
to front have you?

Kind regards
Dave
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Old 30th Sep 2019, 1:31 am   #4
SiriusHardware
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Default Re: Dipole for 10 metres issue.

Also another trick, which fits especially well in lofts, is to slope each 'arm' of a resonant dipole downwards from the feed point so that there is an angle of about 120 degrees between the two arms.

This moves the impedance closer to 50 ohms without changing the lengths of the dipole arms, so the dipole remains at the correct resonant length but has a more transceiver friendly SWR.
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Old 30th Sep 2019, 3:14 am   #5
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Default Re: Dipole for 10 metres issue.

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Originally Posted by SiriusHardware View Post
Also another trick, which fits especially well in lofts, is to slope each 'arm' of a resonant dipole downwards from the feed point so that there is an angle of about 120 degrees between the two arms.

This moves the impedance closer to 50 ohms without changing the lengths of the dipole arms, so the dipole remains at the correct resonant length but has a more transceiver friendly SWR.
Sure that's then not an inverted V?
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Old 30th Sep 2019, 7:26 am   #6
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Default Re: Dipole for 10 metres issue.

I have found a delta loop (triangle each side 3.5 metres long) gives superior results,
you will need a quarter wave length of 75 ohm coax to couple the loop to the 50 ohm
coax. I have even worked U.S. 10m FM repeaters using this.
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Old 30th Sep 2019, 7:34 am   #7
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Default Re: Dipole for 10 metres issue.

Your transmitter and VSWR meter will be running at a lot more power than an antenna impedance analyser. Iffy connections can work on higher power and look open at lower power. Almost shorts can break down under power and yet still allow an antenna to look like a good match at low power.

So have a good check through everything as a first thing.

VSWR meters can also go bad. There are many different types and some are susceptible to knocks, dirty trimpots and switches.

Don't go believing antenna analysers too much either. A UK importer was going to bring in an American unit (no, not MFJ in this case) and the demo unit was made available to Radcom for review. G3SEK was going to write the review, and I was going to do the measurements. Early on in the measurements, I hit trouble. At some phase angles of mismatch the display reading went silly and jumped to the opposite side of the Smith chart. The method it used wasn't a true vector method, but it tried to deduce the vector from multiple amplitude sensors and as the angle approached cardinal points, uncertainty set in.

The unit had a spiffing direct digital synthesiser so it was dead-on frequency though it ate batteries at a prodigious rate. But people would believe what appeared on the LCD, after all, it was digital, wasn't it? But capacitive loads could get displayed as inductive and vice-versa. It was simply unfit for its intended purpose.

If what we found had been written up, the review would have been a stinker. You couldn't tell without other test hear when it was going silly or when it was working, so you could never trust it. The review never got written. The thing had already been on sale in the US for a while. Had no-one noticed?

Moral: Some analysers have real phase detectors and should be fine, but some take short cuts and can be highly misleading. Of course, they don't tell you what's inside the box, they're just antenna analysers, aren't they?

But in your case the VSWR you see isn't bad for a dipole. Maybe there is a frequency difference, but 10:1 is a long way off. Maybe iffy construction or maybe the VSWR meter has a problem?

David
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Old 30th Sep 2019, 10:17 am   #8
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Default Re: Dipole for 10 metres issue.

It seems strange. but an MFJ 259 says it's OK as well, my rig SWR meter says infinite as well as the SWR meter. I've checked all the cables with a Fluke DMM, no oc or shorts.

Wound the power down to 5 watts swept on TX from 26MHz to 30MHz. I can't find a match anywhere!.

Very odd.
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Old 30th Sep 2019, 12:55 pm   #9
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Default Re: Dipole for 10 metres issue.

Quote:
Originally Posted by M3VUV51 View Post
Sure that's then not an inverted V?
Of course it is.
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Old 30th Sep 2019, 1:16 pm   #10
M3VUV51
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Default Re: Dipole for 10 metres issue.

well,ive scrapped that one,now making one from the wire from a motorbike lighting coil,all i can think of is the pvc covering on the speaker wire was loading it and changing the point of resanononance?,nothing as wierd s rf!!.
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Old 30th Sep 2019, 1:33 pm   #11
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Default Re: Dipole for 10 metres issue.

I've found the SWR on dipoles can be well out if in close proximity to walls, or not high enough off the ground - in free space.

If always had great difficulty tuning them, so resorted to using verticals instead.
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Old 30th Sep 2019, 2:00 pm   #12
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Default Re: Dipole for 10 metres issue.

Ideally, you should use a balun to convert the unbalanced Tx output to the balanced dipole. If you don't, then maybe one side of the dipole is connected to the ground system via the metalwork in the tx. In that case, you might as well use a quarter wave tuned against ground.

Cheers

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Old 30th Sep 2019, 3:18 pm   #13
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Default Re: Dipole for 10 metres issue.

Quote:
Originally Posted by Aub View Post
Ideally, you should use a balun to convert the unbalanced Tx output to the balanced dipole. If you don't, then maybe one side of the dipole is connected to the ground system via the metalwork in the tx. In that case, you might as well use a quarter wave tuned against ground.

Cheers

Aub
I think Aub has made one of THE most important points concerning use of dipoles.
As you're making one for 10m, a so-called 'choke balun' can be used. This is simply 8 to 10 turns of feeder coiled up directly at the feed-point.
The view shown was one I used on 40m.
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Old 30th Sep 2019, 3:33 pm   #14
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Default Re: Dipole for 10 metres issue.

Yes 'outer-braid RF' can be an issue with dipoles; it's made worse by any asymmetry of shape/closeness-to-ground in the elements, or asymmetry in the way the feeder runs from the feedpoint for the first few 1/4-wavelengths; we can't always erect a dipole in the perfectly-horizontal-plane-with-the-feeder-dropping-away-perpendicularly-for-several-wavelengths way they're shown in the books.

A common-mode choke, as mentioned (either coiled-up coax, coax wound through a toroid, or - my preferred one - a few of the cylindrical 'fat ferrite beads' usually used for RFI-suppression threaded over the coax) will help manage outer-cable RF.

Also worth noting is that the 'velocity-factor' of the elements - and so the cutting length - will vary from unity if the wire used is insulated; this has in the past caught me out when making 'fan dipoles' out of 300-Ohm feeder.
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Old 30th Sep 2019, 5:00 pm   #15
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Default Re: Dipole for 10 metres issue.

Were you using the same length of co-ax for all the measurements/tests?
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Old 30th Sep 2019, 5:18 pm   #16
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Default Re: Dipole for 10 metres issue.

Without a balun you are not measuring the 'antenna' but the 'antenna plus feeder', which the universe considers to be the antenna. The universe is always right.
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Old 30th Sep 2019, 9:39 pm   #17
James Duncan
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Default Re: Dipole for 10 metres issue.

Quote:
Originally Posted by Aub View Post
Ideally, you should use a balun to convert the unbalanced Tx output to the balanced dipole. If you don't, then maybe one side of the dipole is connected to the ground system via the metalwork in the tx. In that case, you might as well use a quarter wave tuned against ground.

Cheers

Aub
You do not require a balun with a simple correctly cut dipole,the would not fix the problem,
Just use and old mains cable wire, no need for it to be insulated, cut correctly and trim for best swr, real simple, make sure all your connectors and joints are correct.
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Old 30th Sep 2019, 9:47 pm   #18
James Duncan
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Default Re: Dipole for 10 metres issue.

Quote:
Originally Posted by Aub View Post
Ideally, you should use a balun to convert the unbalanced Tx output to the balanced dipole. If you don't, then maybe one side of the dipole is connected to the ground system via the metalwork in the tx. In that case, you might as well use a quarter wave tuned against ground.

Cheers

Aub
No way, just what kind of balun are you going to create to change the 73 ohm variable to 50 ohms.? to much rubbish put around about baluns
Just forget baluns and sort out the antenna.
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Old 30th Sep 2019, 10:35 pm   #19
SiriusHardware
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Default Re: Dipole for 10 metres issue.

The balun idea has merit if the dipole (a balanced antenna) is being fed with co-ax (unbalanced) from the transceiver, which it almost certainly is. The other method stated above in various posts - using some form of 'choke' to separate the outer braid of the coax feeder from the antenna - is also equally valid.

As you say, neither of those methods transform the impedance from 75ohms (ish) to 50 ohms. Forming the dipole as an inverted-V rather than as a 'straight' dipole is the simplest way to do that.

You can also make a 'straight' dipole have a low SWR at the frequency of interest by altering the length of the dipole arms, but if you do that then the dipole is no longer resonant at the frequency of interest. The ultimate aim is for it to be resonant length -and- have an impedance of 50 ohms.
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Old 1st Oct 2019, 3:06 am   #20
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Default Re: Dipole for 10 metres issue.

If the only criterion for an antenna is its feedpoint impedance/VSWR, then a dummy load will be an excellent and very broadband antenna

Matters of feedpoint impedance and common-mode feeder currents due to asymmetry can be handled in various ways.

Plan your antenna for the directionality and angle of radiation you want, subject to available space and environment, then look after whatever impedance it gives you.

In general, you may wish to avoid feed arrangements with common-mode current components on either coax or open feeder, as well as unbalanced impedances on open feeder, as these situations allow local QRM to get in on receive.

David
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