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 End Fed Wire antenna for CB 10m

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GaryWilson
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PostSubject: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeTue Oct 17, 2023 5:12 pm

Hi all, been a while since my last post. Anyway straight to the point: I want to make an endfed wire antenna for as many bands as I can, (I have an old Kenwood receiver), but really I would like it resonant for the CB band here in the UK as Ive just bought an old Rotel CB that I want to use. 
A long wire end fed (which will double up as a washing line so to keep it inconspicuous) will be perfect for my needs. I have a garden about 14m long from the house to the shed, and I can string the line horizontally at a height of around 2m. What im trying to work out it how long should i make the wire ?, and what sort of transformer/balun can I make to make it most effective on CB frequency ? Can any of you knd people send a link or plans ?  

I would ideally like to make this end fed so I can receive a number of bands, ive got about 13m of horizontal space, but ideally I want to optomise it for DX on the CB !  

Im a new starter to this hobby and will be buying an antenna analyser to help. Currently I have a single 2.5mm copper wire about 15m long which Ive simply stuck into my receiver (no balun/transformer ect) and im receiving from as far as Italy, but no CB. So i want to make a proper end fed, cut to the correct length and use a balun/transformer. Thanks for any info it will be well received.
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GaryWilson
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeTue Oct 17, 2023 6:13 pm

Hello Jimmy,

The length should be  16.98 feet for a half wave end fed. Leave a little extra for tuning and if you're feeding with coax you would use a 49:1 UNUN not a BALUN.

All the best of luck let us know how you get on. Here's a link to a calculator.   https://calculator.academy/efhw-antenna-calculator/

73's Gary.
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeTue Oct 17, 2023 6:46 pm

Thanks Gary thats useful.

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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeTue Oct 17, 2023 6:51 pm

You're welcome Jimmy. 

I calculated that for 27.555 so should be okay for the C.B. freeband and a fair bit of 10 meters as well. Your analyser will be your friend here. Have fun!

73's Gary.
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeTue Oct 17, 2023 6:55 pm

Yep i will be making sure its done as best i can. Just thinking, would I get better performance on CB out of an efhw cut just for 27.55 Mhz, rather than 80m and relying on the harmonics (is that the correct term) for the CB bands ?
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeFri Oct 27, 2023 8:59 pm

I have built one of these and found as you move higher up the band in frequency the more receive noise you get. Found it works ok on the lower bands but at 27/28mhz, it has a very high noise floor compared to say a cut hoz dipole for the frequency. It works, not as good as other antennas but in your case better than nothing.
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeSat Oct 28, 2023 1:44 am

JimmyD wrote:
Yep i will be making sure its done as best i can. Just thinking, would I get better performance on CB out of an efhw cut just for 27.55 Mhz, rather than 80m and relying on the harmonics (is that the correct term) for the CB bands ?

An EFHW cut for 80m is around 40m long, so that isn't going to fit the space you said you had.  

Also, the CB band is not harmonically related to the amateur bands, so an EFHW cut for 80m (Or 40m or 20m) won't have a decent tune on 11m regardless.

However, as you only want it for transmitting on 11m, tune it for that and it'll work OK as a receive antenna elsewhere.

Just bear in mind that a horizontal antenna on that frequency isn't going to be very good for local stuff.
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeSat Oct 28, 2023 4:11 pm

Thanks for the responses. Ive been waiting more than a week for a 49:1 unun to arrive in the post, still not arrived ! Anyhow when this arrives I will cut the long lead to various lengths and see which works best for receiving the 11m band. I now also have a mini13000 nano-VNA so can test with that, although it probably wont be too critical to have a low SWR for receiving only. 

Right now in using an inverted dipole strung across bedroom ceiling and do receive CB on my receiver from about 30 miles at best (I guess thats someone transmitting on 100w), and often hearing local CB easily. 

I tested my dipole with the VNA and im getting a v.high SWR, no matter how I configure it im not getting below 8:1. This got my thinking if I coil the coax to act as a choke (as near to the dipole centre as I can) then this should lower the SWR right ? Also when testing a dipole for SWR, would test results change if moved to another location and/or if the legs were inverted or T shaped ?
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeSat Oct 28, 2023 11:30 pm

Remember, SWR is only an indication of the impedance match, and that is really only important for maximum power transfer from radio to antenna system, and much less relevant for receive.

A choke will rarely do anything to lower SWR, unless it is needed in that particular type of antenna design.

On a dipole, the feedpoint impedance will vary based on height above ground, as well as configuration, though 8:1 suggests something is very wrong.

As you have a VNA, is the 8:1 the lowest at any frequency, or just on 27MHz somewhere, and what are the impedance measurements (R and X) at that point?
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeMon Oct 30, 2023 9:13 pm

Hi wish I had responded sooner but been away working. Anyhow since my last post Ive moved my dipole to a different position on the ceiling, now both legs are at a smaller angle of about 30 deg, like a thin slice of pizza. 

So measurements from my VNA @ 27.50000 MHz is now 3:1 (better than before), with an Rx value of 19.52, and an Xs value of 22.08, there's also an Ls value (that's losses right ?) of 0.13 micro henrys. 

If I do a sweep of the Ham bands (3 MHz - 33MHz) the only dips to low SWR are at 16.1000 MHz (1:8 SWR), and at 27.5000 MHz (3:1 SWR).  So I'm getting a decent SWR at 16 MHz, but this is not a ham band is it ? Anyhow does all this sound normal as I'm still a novice with this but learning slowly. Thanks
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeTue Oct 31, 2023 7:38 am

JimmyD wrote:

So measurements from my VNA @ 27.50000 MHz is now 3:1 (better than before), with an Rx value of 19.52, and an Xs value of 22.08, there's also an Ls value (that's losses right ?) of 0.13 micro henrys. 

No, "L" is Inductance, in Henry's - part of the reactive (X) component of your complex impedance.

Without going into too much detail, if you were chasing resonance (and people get way too hung up on resonance, often without understanding what it means) then you would want to see a value for R and nothing for X.

If you wanted a low SWR, you would want to see R+X as close to 50 ohms as possible.

A dipole's tuned frequency is set primarily by the length of the wires, but it's impedance comes from things like height about ground and how you have it setup (flat top, inverted v, angle of the v), etc.

If the dip is at 27.something MHz then you have the length roughly right, but your impedance being nowhere near 50ohms is causing the higher SWR.

However, a better dip at 16MHz is interesting - not a ham band, but that's not really relevant.  How long is each leg of the dipole?
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeTue Oct 31, 2023 11:41 am

does resonance mean some sort of vibration  ?
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeTue Oct 31, 2023 11:57 am

skyrider wrote:
does resonance mean some sort of vibration  ?

With antennas, it simply means where the load impedance is purely resistive, with no reactive components.  

From which you can see it has nothing to do with SWR or radiation efficiency.
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeWed Nov 01, 2023 4:53 am

JimmyD wrote:
Hi wish I had responded sooner but been away working. Anyhow since my last post Ive moved my dipole to a different position on the ceiling, now both legs are at a smaller angle of about 30 deg, like a thin slice of pizza. 

Yeah you can't bend it that tight. You can't have any angle less than 90 degrees either between the legs or when bending the legs of a dipole.
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeWed Nov 01, 2023 8:50 pm

However, a better dip at 16MHz is interesting - not a ham band, but that's not really relevant. How long is each leg of the dipole? wrote:
One leg is 2.41 m, the other is around 2.5m

but your impedance being nowhere near 50ohms is causing the higher SWR. wrote:
Great, so I guess that means I need to lower the impedance; how would I do that ??
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeWed Nov 01, 2023 8:53 pm

Northern Crusader wrote:
JimmyD wrote:
Hi wish I had responded sooner but been away working. Anyhow since my last post Ive moved my dipole to a different position on the ceiling, now both legs are at a smaller angle of about 30 deg, like a thin slice of pizza. 

Yeah you can't bend it that tight. You can't have any angle less than 90 degrees either between the legs or when bending the legs of a dipole.

It seems better at receiving while at this smaller angle than it does in a L shape (along the corners of room). I expect there's alot of variables that need some consideration; anyhow its all a learning curve.
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PostSubject: Re: End Fed Wire antenna for CB 10m   End Fed Wire antenna for CB 10m Icon_minitimeThu Nov 02, 2023 3:11 am

JimmyD wrote:
However, a better dip at 16MHz is interesting - not a ham band, but that's not really relevant. How long is each leg of the dipole? wrote:
One leg is 2.41 m, the other is around 2.5m

but your impedance being nowhere near 50ohms is causing the higher SWR. wrote:
Great, so I guess that means I need to lower the impedance; how would I do that ??

OK, first thing is to get both legs equal length, and the v out to ideally more than 90 degrees.  Both of those could be helping cause the weird dip at 16MHz, and they certainly won’t be helping the SWR.  See where you are at when thats done.

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