5/8 Wave vertical antenna question

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5/8 Wave vertical antenna question

Post by 3metrejim » Mon Sep 17, 2018 8:00 pm

I once made a successful 5/8 wave over 5/8 wave co-linear antenna, with groundplane (3 radials). The thing I couldn't understand was the base matching. I had 5 turns on an approx 28mm former spread over about 40mm. The coil was tapped at around 70% up. I couldn't get a perfect match without adding a capacitor across the coil - turned out to be around 10pf (it was one of those small green trimmers about half mesh).

What I'd like to know is: Is it possible to make get the antenna working as well without the capacitor, as with it the power rating would be pretty limited - I once put 30 watts into it without the trimmer melting, but I don't think it could handle much more. I was thinking a small length of coax as a small capacitor, but that wouldn't be very adjustable.

Is the coil/capacitor combination working like a 1/4 wave (tapped) matching section would (form the tap point, it calculates to around the 140 ohms mark for the radiating element), or is something else happening?

The antenna was based on the one in the VHF/UHF manual (4th edition) page 8.43 fig 108b for anyone with the book (used to be the pirates bible!). Matched bottom section first then added top with no change in SWR, but an increase in signal at a 6 mile distance, so no doubt that it did work properly. It also looks almost identical to a silver rod CB antenna - and it's almost the same length (pretty huge), but you can take off the top and use it as a plain 5/8 wave if required.

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Re: 5/8 Wave vertical antenna question

Post by 3metrejim » Mon Sep 17, 2018 8:15 pm

Here is a picture of it, taken some years ago now. As you can see it's rather long (and a bit bendy in storms).
antenna-full.jpg
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Re: 5/8 Wave vertical antenna question

Post by radionortheast » Tue Sep 18, 2018 1:35 pm

I should know how capacitors work but don't suppose someone else might be-able to say maybe using afew would increase power handling?

long poles are hard to get working, suspect when you added the radials it changed the angle of radiation

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Re: 5/8 Wave vertical antenna question

Post by yellowbeard » Tue Sep 18, 2018 5:33 pm

There is a large thread on here about the NRG high gain vertical, the instructions are for download on page 3:

Code: Select all

http://www.radionecks.co.uk/viewtopic.php?f=11&t=231
This uses metal plates moved in and out by nylon bolts as a capacitor, the voltages at the base are large - one of the guys managed to melt the granny out of the aluminium element - the capacitor still looked fine.
Here is a link for calculating the capacitor value - bear in mind a larger gap between the plates gives higher power handling but requires larger plates:

Code: Select all

https://daycounter.com/Calculators/Plate-Capacitor-Calculator.phtml
:tup

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Re: 5/8 Wave vertical antenna question

Post by 3metrejim » Tue Sep 18, 2018 5:59 pm

The high voltage thing and difficulty tuning put me off the half wave vertical. I found that the 5/8 wave I made worked very well and the ground plane had beneficial effects. As you can see it was made out of an old CB anntenna (Who else did that ???) - the antenna in my picture is now over 20 years old (and I still have it - in pieces), it was built as a test antenna.

I have to admit though, that the dreadful ar****f 5/8 wave thing was something that I had suggested to him to make - unfortunately me being very young at the time, I was not taken seriously enough to be listened to (by the guy who knew better), that the antenna rod protruding through the bracket on a CB aerial was way too long (too much capacitance) to work at band II - you could make it work by using the bracket as a holder and ground plane attachment though. I used a disc on a bolt that you could screw in and out as an adjustable capacitor on one of those (as the adjustment point was grounded, you could tune with power applied). It worked well, but I never understood the matching 100%, which is why I was asking for an explanation of exactly how it works.

I was convinced, at the time, that it was probably as good with 30 watts as someone putting 100 Watts into a dipole - the signal from it was receivable at over 8 miles distance with a puny 10 watts into it - there was no doubt that it worked well.

I was thinking that the coil/capacitor combination could be changed into a rolled up 1/4 wave? match - a little like a J-pole but with 2 side by side wound coils with their common bottom points connected to the ground plane (alternatively viewed like a single coil but with capacitance coming from the second winding that is unconnected at it's top end - an untwisted gimmick capacitor/coil combination?) - something I've never seen, but seems like it might work. Albert? Anyone technically minded?

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Re: 5/8 Wave vertical antenna question

Post by 3metrejim » Tue Sep 18, 2018 9:26 pm

I've just measured the top element and worked out that it is 0.6 wavelength (90% of a 5/8). I do not know whether this is due to the largeish diameter tubes or whether I made a mistake somewhere. Also checked the coil tapping and it indicates an end impedance for the radiator of around 100 ohms - does this sound right for a 5/8 wave element over a ground plane, or did I manage to construct something half way between a 1/2 wave and a 5/8 wave antenna?

I know it worked well, and the lower impedance (compared to a half wave) means there should be less of a high voltage problem at the bottom of the antenna element.

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Re: RE: Re: 5/8 Wave vertical antenna question

Post by Electronically » Wed Sep 19, 2018 2:58 pm

yellowbeard wrote:There is a large thread on here about the NRG high gain vertical, the instructions are for download on page 3:

Code: Select all

http://www.radionecks.co.uk/viewtopic.php?f=11&t=231
This uses metal plates moved in and out by nylon bolts as a capacitor, the voltages at the base are large - one of the guys managed to melt the granny out of the aluminium element - the capacitor still looked fine.
Here is a link for calculating the capacitor value - bear in mind a larger gap between the plates gives higher power handling but requires larger plates:

Code: Select all

https://daycounter.com/Calculators/Plate-Capacitor-Calculator.phtml
:tup
Thanks yellow beard but see how in his instructions he shows a screwdriver for winding the coils but trouble is screwdrivers come in certain diameters .I know he says he puts tape on it but that still doesn't help .And I never found lees instructions very clear .It was like it was cutting corners or either that he wasn't sure himself how it was made .And plus he doesn't state the length and width of the capacitor plates .Or either that I'm missing a page lol .

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Re: RE: 5/8 Wave vertical antenna question

Post by Electronically » Wed Sep 19, 2018 3:43 pm

3metrejim wrote:I once made a successful 5/8 wave over 5/8 wave co-linear antenna, with groundplane (3 radials). The thing I couldn't understand was the base matching. I had 5 turns on an approx 28mm former spread over about 40mm. The coil was tapped at around 70% up. I couldn't get a perfect match without adding a capacitor across the coil - turned out to be around 10pf (it was one of those small green trimmers about half mesh).

What I'd like to know is: Is it possible to make get the antenna working as well without the capacitor, as with it the power rating would be pretty limited - I once put 30 watts into it without the trimmer melting, but I don't think it could handle much more. I was thinking a small length of coax as a small capacitor, but that wouldn't be very adjustable.

Is the coil/capacitor combination working like a 1/4 wave (tapped) matching section would (form the tap point, it calculates to around the 140 ohms mark for the radiating element), or is something else happening?

The antenna was based on the one in the VHF/UHF manual (4th edition) page 8.43 fig 108b for anyone with the book (used to be the pirates bible!). Matched bottom section first then added top with no change in SWR, but an increase in signal at a 6 mile distance, so no doubt that it did work properly. It also looks almost identical to a silver rod CB antenna - and it's almost the same length (pretty huge), but you can take off the top and use it as a plain 5/8 wave if required.
most likely your coil Is 438 nh or close .Which basically is tuned for 72mhz at 10 pf capacitor .Are trying trying to tune this for what frequency ?

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Re: RE: Re: RE: 5/8 Wave vertical antenna question

Post by Electronically » Wed Sep 19, 2018 4:44 pm

Electronically wrote:
3metrejim wrote:I once made a successful 5/8 wave over 5/8 wave co-linear antenna, with groundplane (3 radials). The thing I couldn't understand was the base matching. I had 5 turns on an approx 28mm former spread over about 40mm. The coil was tapped at around 70% up. I couldn't get a perfect match without adding a capacitor across the coil - turned out to be around 10pf (it was one of those small green trimmers about half mesh).

What I'd like to know is: Is it possible to make get the antenna working as well without the capacitor, as with it the power rating would be pretty limited - I once put 30 watts into it without the trimmer melting, but I don't think it could handle much more. I was thinking a small length of coax as a small capacitor, but that wouldn't be very adjustable.

Is the coil/capacitor combination working like a 1/4 wave (tapped) matching section would (form the tap point, it calculates to around the 140 ohms mark for the radiating element), or is something else happening?

The antenna was based on the one in the VHF/UHF manual (4th edition) page 8.43 fig 108b for anyone with the book (used to be the pirates bible!). Matched bottom section first then added top with no change in SWR, but an increase in signal at a 6 mile distance, so no doubt that it did work properly. It also looks almost identical to a silver rod CB antenna - and it's almost the same length (pretty huge), but you can take off the top and use it as a plain 5/8 wave if required.
most likely your coil Is 438 nh or close .Which basically is tuned for 72mhz at 10 pf capacitor .Are trying trying to tune this for what frequency ?

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If its for around 73mhz I would use 47pf with 100 nh coil which would give around 45 ohms .But keep same diameter but less turns at say around 2.27 turns .

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Re: 5/8 Wave vertical antenna question

Post by 3metrejim » Wed Sep 19, 2018 5:59 pm

Thanks electronically. The Antenna did tune to Band II frequencies so I may have been out a little with the capacitor value (I had to guesstimate as the tiny trimmer managed to detatch itself, and was lost, at some point while transporting the antenna pieces.

I've done a little research and it turns out that my design is quite similar to the 2m Diamond antenna and someone has upgraded one with co-axial cable capacitors - see here https://pa0fri.home.xs4all.nl/Ant/X510N ... cation.htm so I'll go down that route and clip the cable off bit by bit to get a good match. Still not easily adjustable though - has to be adjusted for a set frequency - maybe I can use a bolt-in-a-tube type (co-axial) adjustable capacitor for up to around 15pf; I was thinking of using polypropylene dielectric (doesn't seem to absorb RF when given the microwave oven test).

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Re: 5/8 Wave vertical antenna question

Post by 3metrejim » Wed Sep 19, 2018 6:43 pm

This link below details construction of the Diamond X200N which is almost identical in electrical construction to my version, ignoring the fact that it is designed for the 2m/70cm amateur bands. Mine differed by having no series capacitor on the input.

http://www.dl2jas.com/antennen/x200n/x200n.html (use google translate if required)

I still do not completely understand in detail how the base matching works / what impedance a 5/8 wave element over a ground plane has.
It would be nice to know those details so that a technically correct design can be done, even though I have one that seems to work well. I couldn't justify the expense of an antenna analyser just to satisfy my curiosity, unfortunately.

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Re: 5/8 Wave vertical antenna question

Post by Electronically » Thu Sep 20, 2018 11:53 am

I could be right I could be wrong .Here goes anyways, the 3 radials horizontal I think is 1/8 between them .And the vertical poles from above the bottom coil to the middle is a 5/8 then as we look above the middle coil there is another 5/8 .I could be wrong .

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Re: RE: Re: 5/8 Wave vertical antenna question

Post by Electronically » Thu Sep 20, 2018 12:00 pm

Electronically wrote:I could be right I could be wrong .Here goes anyways, the 3 radials horizontal I think is 1/8 between them .And the vertical poles from above the bottom coil to the middle is a 5/8 then as we look above the middle coil there is another 5/8 .I could be wrong .

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1/8 wavelength section to give a matching section a better impedance to chew on I think

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Re: RE: Re: RE: Re: 5/8 Wave vertical antenna question

Post by Electronically » Thu Sep 20, 2018 2:03 pm

Electronically wrote:
Electronically wrote:I could be right I could be wrong .Here goes anyways, the 3 radials horizontal I think is 1/8 between them .And the vertical poles from above the bottom coil to the middle is a 5/8 then as we look above the middle coil there is another 5/8 .I could be wrong .

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1/8 wavelength section to give a matching section a better impedance to chew on I think

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I'm going to modify a antron 99 antenna they look simple once I get my hands on one .When I do I'll post on this

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Re: 5/8 Wave vertical antenna question

Post by 3metrejim » Thu Sep 20, 2018 10:23 pm

Electronically - Not sure if a mod of an antron 99 will work. I've found the ground plane to be quite important so that changes in mounting and cabling don't upset the match of the antenna.

I managed to (sort of - the software is very clunky) simulate my antenna with EZNEC (quite an apt name for this message board!) but found it to give very granular results, due to (I think) the segment limit in the demo version.

Anyway, the simulation shows an impedance of somewhere around the 110 ohm level for the radiator with a high value of -j (capacitative reactance).

So this is how the match seems to be working: The coil on it's own, with the tap, provide a match from 50 ohm to the approx 110 ohm of the antenna element while the antenna capacitance resonates with the coil at a frequency slightly above what you would like. An additional capacitor then brings the resonant frequency down to where you want it. The capacitative reactance of the antenna is absorbed into the tuning, leaving a resistive match between the 50 ohm feed and the antenna element, which the coil tap provides.

Looks like I may have anwsered my own questions, but someone please correct me if I'm obviously wrong.

When I first made that antenna it was books(!) only and a lot of experimenting. No internet, no pictures of the internals of professionally made antennas, and no simulators that you could use at home.

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Re: 5/8 Wave vertical antenna question

Post by 3metrejim » Mon Sep 24, 2018 1:12 am

Just had a look at the a****f website at the 5/8 wave antenna there - in production since 1996 (is that a coincidence? :) see above).

After some more online digging, the match on mine looks like something called a 'hairpin' match - except mine is rolled into a coil and tapped. A****f version uses a more conventional 'hairpin' used as a tunable inductor, I rather use a tunable capacitor although that could limit the maximum power usable depending on construction. The a****f version has what looks like too short (and poorly secured) groundplanes (tried exactly the same with modified CB antenna and after a while of wind vibration, the groundplanes became loose) - can't see how a 50 ohm match is gotten either, mine has to tap the coil, the hairpin equivalent.

Maybe I should phone again, and ask for royalties, as was based on my idea of re-purposing a cheap CB antenna (10% should be plenty :rlol )

Seen reports on here that the a****f version doesn't work well - and the price!. Mine did work well, and likely still costs a lot less to make especially if you can construct a decent groundplane fixing method and a higher powered tuning capacitor. Instead of the CB antenna you can use a 1 inch dia aluminium tv mast for the first part of the element and another tube that just slots inside; the only reason I initially used an old CB antenna - remember there wasn't good internet access back then and ebay had only recently launched, also CB antennas were cheap - not so much nowadays.

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Re: RE: Re: 5/8 Wave vertical antenna question

Post by Electronically » Mon Sep 24, 2018 12:47 pm

3metrejim wrote:Just had a look at the a****f website at the 5/8 wave antenna there - in production since 1996 (is that a coincidence? :) see above).

After some more online digging, the match on mine looks like something called a 'hairpin' match - except mine is rolled into a coil and tapped. A****f version uses a more conventional 'hairpin' used as a tunable inductor, I rather use a tunable capacitor although that could limit the maximum power usable depending on construction. The a****f version has what looks like too short (and poorly secured) groundplanes (tried exactly the same with modified CB antenna and after a while of wind vibration, the groundplanes became loose) - can't see how a 50 ohm match is gotten either, mine has to tap the coil, the hairpin equivalent.

Maybe I should phone again, and ask for royalties, as was based on my idea of re-purposing a cheap CB antenna (10% should be plenty :rlol )

Seen reports on here that the a****f version doesn't work well - and the price!. Mine did work well, and likely still costs a lot less to make especially if you can construct a decent groundplane fixing method and a higher powered tuning capacitor. Instead of the CB antenna you can use a 1 inch dia aluminium tv mast for the first part of the element and another tube that just slots inside; the only reason I initially used an old CB antenna - remember there wasn't good internet access back then and ebay had only recently launched, also CB antennas were cheap - not so much nowadays.
I did have a look at the venom 1/2 wave antenna it never had ground planes .When I removed the bottom part of antenna all there was inside was a coil wound on a cardboard tube .Couldn't see any capacitor thought .It put me in mind of the bazooka 1/2 antenna which was only a coax loaded coil no capacitor .aereff antennas never change they still keep same design as it was when it was veronica .Your right about thee ground planes they come loose when wind hits the ground planes .

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Re: RE: Re: RE: Re: 5/8 Wave vertical antenna question

Post by Electronically » Mon Sep 24, 2018 12:56 pm

loading coil should've said a balun coil .
Electronically wrote:
3metrejim wrote:Just had a look at the a****f website at the 5/8 wave antenna there - in production since 1996 (is that a coincidence? :) see above).

After some more online digging, the match on mine looks like something called a 'hairpin' match - except mine is rolled into a coil and tapped. A****f version uses a more conventional 'hairpin' used as a tunable inductor, I rather use a tunable capacitor although that could limit the maximum power usable depending on construction. The a****f version has what looks like too short (and poorly secured) groundplanes (tried exactly the same with modified CB antenna and after a while of wind vibration, the groundplanes became loose) - can't see how a 50 ohm match is gotten either, mine has to tap the coil, the hairpin equivalent.

Maybe I should phone again, and ask for royalties, as was based on my idea of re-purposing a cheap CB antenna (10% should be plenty :rlol )

Seen reports on here that the a****f version doesn't work well - and the price!. Mine did work well, and likely still costs a lot less to make especially if you can construct a decent groundplane fixing method and a higher powered tuning capacitor. Instead of the CB antenna you can use a 1 inch dia aluminium tv mast for the first part of the element and another tube that just slots inside; the only reason I initially used an old CB antenna - remember there wasn't good internet access back then and ebay had only recently launched, also CB antennas were cheap - not so much nowadays.
I did have a look at the venom 1/2 wave antenna it never had ground planes .When I removed the bottom part of antenna all there was inside was a coil wound on a cardboard tube .Couldn't see any capacitor thought .It put me in mind of the bazooka 1/2 antenna which was only a coax loaded coil no capacitor .aereff antennas never change they still keep same design as it was when it was veronica .Your right about thee ground planes they come loose when wind hits the ground planes .

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Re: 5/8 Wave vertical antenna question

Post by Electronically » Mon Sep 24, 2018 1:55 pm

Could always make a 2 pole vertical or a 3 pole vertical without ground planes .I'll start of with the 3 pole vertical .Bottom pole would be a 1/4 pole then apply coil at top then apply 1/2 pole on top of it then apply a second coil on top of it .Then apply a second 1/2 pole on top the coil .
For the 2 pole element which uses only one coil in between the two pole vertical.
1/4 pole then coil on top if it then 1/2 on top it.


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Re: 5/8 Wave vertical antenna question

Post by 3metrejim » Mon Sep 24, 2018 7:38 pm

Electronically, it will make your life easier if you start with a single pole antenna and get that to work first. Once you get that working properly then adding to it will be quite easy.

Have a look at the link I posted in 'crucial links' - I managed to find the diagrams online that mine was based on. Hopefully, the link still works.

viewtopic.php?f=21&t=2042

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