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 I can't get my head round... 
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Post I can't get my head round...
Well I'm still blundering along with the fishing rod antenna as I dare not stick anything else up as even her in drawers is digging her feet in now.

Anyway this antenna is a length of fat coax braid flattened to about 13mm wide and wrapped in PVC tape from top to bottom to the outside of my fishing rod which for the sake of one less red herring we'll say is fibreglass.

The antenna is ground mounted with about 30 odd radials many of which are about 6m long but at least 4 are 10m long all radials are now about 10mm underground thanks to the worms.

The fishing rod is 9m long with a length of straight single core copper 4mm2 wire sticking up another 600mm at the top where the pole was too blasted short for 40m :blackeye: At the feedpoint I have a 1:1 current balun for 40m from Steves chart on his Karinya page. :good:

Back to my latest farting around and I was thinking of replacing the fishing rod with thin alloy tubing so that I could slide the sections in and out to tune for resonance rather than simply use the ATU in the house (MFJ 986 differential T) however following the recent 5/8th wave 10dBd gain thread :blush: and the dimension on 17M not being far off the length of this 40m 1/4 wave vertical I thought I might tune out the reactance by winding a coil and shoving it across the feed point.

I hasten to add I'm feeding the antenna with my 259B so there won't be a fried radio in the for sale section next week, anyway, I took readings on three bands and I'm none the wiser as to what this blinking thing is doing, on 40m I get a best figure on 7.0164 MHz of a 1:1 SWR, an R of 43 ohms and X= 4, at 21.472 MHz the readings are 1:1 SWR, an R of 58 ohms and X=0.

Now comes the weirdy bit as the physical size of the antenna should be close to a 5/8th wave on 17 and should be a piece of cake to match to 50 ohms according to the nonsense I've read on the web, however starting at 18.101MHz I get an SWR of 12:1 an R of 3 ohms with X equalling 4, obviously as winding a coil is easy the first thing I did is wound a coil... then another coil, followed by another coil not forgetting to stretch and compress them to increase/decrease the inductance as well as winding these things on different diameter formers.... basically I know that I need a tuned circuit that can be brought in and out of circuit when required but what sort of sadist publishes something on the internet along the lines of "all that is required is a simple coil to bring the antenna back to resonance" especially when they know I avoid capacitors like the plague?

In morons terms what ghetto products can I cobble together at the feed point to add when working 17M and then remove when working 40 and 15?

Rewinding a second though and that 15M antenna is a 3/4 wavelength currently and all my signal is going too high.... would it be easier to use alloy tube and have one matching network for 5/8th waves on 15 and 17 at the feedpoint and then slide the tube of the antenna in and out?

I'm going to bed with an headache now. :blink:

Cheers, Dave.


Fri Jul 10, 2015 10:23 pm
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 I can't get my head round... 
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Post Re: I can't get my head round...
Where were you measuring the impedances - right at the base of the vertical, or through a length of coax?

Steve G3TXQ


Sat Jul 11, 2015 7:54 am
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 I can't get my head round... 
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Post Re: I can't get my head round...
Right at the base as I put an SO239 on the input of the current balun enclosure, I thought measurements there would take out any magical effect the coax feed length might add.

Realistically Steve could a network at the feed point to match a 5/8th dimension antenna be the same for 15 and 17 if I used telescopic tubing to physically change the antenna length or would the frequency change always alter the impedance required as in a normal reactance calculation is. xl = 2 pi fl?... last time I did any calculating like this was back in the late 70s on my electrical apprenticeship at Booth Lane tech!

Cheers, Dave


Sat Jul 11, 2015 8:15 am
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 I can't get my head round... 
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Post Re: I can't get my head round...
The 40m and 15m impedances look pretty much what you would expect, but the 17m figures are very odd! You'd expect something like R=100 and X=-450.

If we ignore your analyser readings and go with the theory, the inductor should be about 4uH.

No, you can't use a simple matching network for the two bands - the component values you require scale directly with frequency.

Steve G3TXQ


Sat Jul 11, 2015 9:26 am
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 I can't get my head round... 
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Post Re: I can't get my head round...
Thank you Steve but just to clarify would a single 4 mu Henry coil across the feed point bring my 9.6m long vertical back inline on 17m?

Also the readings on my 259b seem about right on 40 and 15 allowing for the the compromise of working both bands on one piece of wire however why are my 17m readings so at odds with an R of 3ohms and an SWR at 12:0, am I looking at a reading in the wrong part of 17, I took the reading there as the reactance value at 4 seemed about the best it was going to be sweeping back and forth across the spectrum?.. apologies for being genuinely thick. :blink:

Cheers, Dave.


Sat Jul 11, 2015 1:27 pm
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 I can't get my head round... 
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Post Re: I can't get my head round...
I've no idea why your 17m readings are so odd!

EZNEC predicts that a vertical which is resonant on 21.47MHz (R=55, X=0) will also be resonant at 7.04MHz (R=37, X=0). Those figures ignore ground losses, so your measured results look about right.

But on 18.1MHz the impedance should then be R=100, X=-450, that's an SWR of 44:1.

A coil of 4uH will cancel that reactance, and you should then be able to find a tap position where you get a 50 Ohm match.

Steve G3TXQ


Sat Jul 11, 2015 3:31 pm
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 I can't get my head round... 
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Post Re: I can't get my head round...
Only had a short amount of time to fiddle with the antenna but did disconnect the existing radial field at one point and run a couple of 17m half wave length radials out instead at about 150mm above ground and that threw everything out with the antenna barely resonant anywhere, the best readings were probably at 22 MHz, 40m was miles out.

Having returned the antenna back to vertical and radial field again I found a couple of additional points at 35.805 MHz and 50.088 where the SWR and R were in the region of 1:1 at 50 odd ohms and reactance was in single figures, both points were fairly narrow though and 50 MHz in particular would be a tease as it's quite out of tune by the time you get to the phone section.

Just a thought and could it be that my current balun has failed as it would have had the best part of 400 watts thrown at it in the past when I used to use my FL2100B, the cable looks intact wound around the toroid but as the coax is standard RG58 there's a chance the centre core might migrate due to heating possibly?

Either way the antenna never shows an R of more than single figures at any point other than those mentioned so far on 7, 21, 35.8 and 50.088 MHz.

Another thought and I'm only using about 2 turns on the ATUs inductor but have the capacitor nearly fully meshed.

I used to get completely non text book readings when going out mobile on HF too so maybe I'm putting too much faith in the 259B analyser again however as nothing has ever blown and I've worked some distant stuff I was hoping that I was thereabouts with the setup.

Cheers, Dave.


Sat Jul 11, 2015 6:37 pm
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