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5/8 wave coil

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  #9  
Old 08-18-2009, 02:58 PM
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thanks Doc,

reason I asked that is because any vertical I have disected uses the "tapped coil" method, and also I've noticed the specs on antenna's seem to make a point of mentioning that they are DC grounded to prevent static buildup. I've even seen claims by people on other forums that a build up of static on your antenna can even attract lightning If this is true or not I don't know, but I cant really think of any reason why it would be.

I've never had a problem with static (that I know of) so I'll go for the series inductor method, it will make things easyer too, rather than having to remove the enamel from the wire to relocate the tap I can just start with a long coil and shorten the length until i get a good match. I'll be using this antenna for /p anyway so it won't be up long enough for static to be an issue.

Simon

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Old 08-20-2009, 04:18 PM
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After some discussion in another thread (in the ham antenna forum), I realized what was more likely happening with the simpler base load on my supposed 5/8 wave antenna, even though it seemed to work well...

I figured out it was a base loaded 3/4 wave antenna!

I'm not sure what freq your antenna is designed for, but if I calculate the length of a 3/4 wavelength wire for 27 MHz, I get 26 feet.

If I take your 22.8 length and add the length of the coil you made (9 turns around 1.5 inch diameter = 42"), I get 26.33 feet...looks like a 3/4 wave antenna, but someone will point out that is not really true either.

HiDef also mentioned the the concept of their being an optimum length-to-diameter ratio for the coil, which I believe is also important. W8JI also mentioned this in one of his articles.

Inductors

Lots of other good reading there too...

Anyway, in my case I'm going to rebuild the classic feed as it was, maybe changing the materials a bit.

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  #11  
Old 08-20-2009, 07:15 PM
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C2,
I think the gist of what you said is correct, but I also think that one or two of the 'steps' along the way are not correct.
Yes, a loading coil does change an antenna's effective 'electrical' length. The amount of inductive reactance furnished by that loading coil and the length of the radiator above it (and/or below it) will determine what that effective 'electrical' length will be.
No, the length of the conductor that makes up that loading coil has no direct relation to the resulting effective 'electrical' length. You can't just pull that coil out straight and add it's length to the length of the rest of the antenna and come up with the answer to what that 'electrical' length of antenna is. I'm sorry, but it just doesn't work that way.
That's also why there can be "an optimum length-to-diameter ratio for the coil", which means producing the 'right' amount of inductive reactance in the most efficient way.
In general, it means that 'short + fat' coils with adequate turn spacings are 'better' than 'tall + skinny' coils with adequate turn spacings. And like all generalizations, there will be particular circumstances where that is not true because of some extenuating 'other' things involved in the whole thing. (And just like you predicted, somebody pointed that out as not being 'right'. But hey, everybody get's 'lucky' now and then! But then, who say'z it's really a 3/4w length equivalent?? I wish that were true, it'd make things soooo much easier.)
What makes all this so much fun is that while what you originally set out to do may have failed, you find out so many other useful thingys that it makes the whole thing worth while. So, 'mistakes' are to be expected. They should be counted as a learning experience and remembered (so you don't do the same thing again, or quite so often).
- 'Doc

(That also explains the knots you see on antenna people's heads. That muttering you hear isn't formulas being raced through to check their accuracy, it's the guy saying, "I know better than that. Why'd I do that? Aw 'spit'!...did it again!", and so on. While they are banging their head against a wall.)
Who, me? I ain't never!

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Old 08-20-2009, 07:23 PM
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Quote:
Originally Posted by multimode200 View Post
9 turns of 1.5mm wire on the 1.5 inch diameter former. The .64 antenna which I build tuned in at 1.2:1 on the third turn from the bottom. My .64 is 22.8 feet in length and performs better than a shortened 5\8ths without a doubt. I used four 1\8th wave radials on my .64 though and the coil was soldered after I fitted them. You must remember that when you solder your tapping point, make sure your radials are already connected otherwise it will cause problems with your SWR if you fit them afterwards.
thanks for that bud, it gives me a good starting point. At the moment I have the main element I'm going to use, which is a shade under 22 feet in length, rather than change the bottom part of the half wave silver rod for a longer piece, I changed the top piece for half of the reflector from a 10M yagi that I've had sitting in my shed for a few years now, which gives me the length I need. I've used a piece of pine dowel to isolate the main element of the antenna from ground, and a 1.5 inch plastic tube (from one of my kids toys that they don't use) to wind the coil onto. The coil is at the moment about 15 turns, I found a piece of 16 guage antenna wire and wound it all on, with the intention of trimming it down untill I get a good match.

For the ground plane I'm using some more of the 10M yagi aluminium tubing, which is 4 foot in length, but I am also adding another 5 foot of copper wire and attaching it to the next radial like you did on your half wave vertical. I'm using 4 radials so the angle for the wire won't be as accute as it would be only using 3 ground plane legs.

I must admit I am a little confused now from the last post, allthough I understand that the matching coil on a 5/8th wave antenna does not replace the missing element length between a 5/8th and a 3/4 wave, the added inductance does simulate the antenna to give the same impedance match as a 3/4 wave would do.

So if you have a 5/8th wave element with a base loaded coil (no tap), it then becomes a 3/4 wave base loaded antenna, but if you have a 5/8th wave element with a tapped coil it is a 5/8th wave antenna? It might just be me, but that is how I read C2's post.

Quote:
Originally Posted by C2 View Post
After some discussion in another thread (in the ham antenna forum), I realized what was more likely happening with the simpler base load on my supposed 5/8 wave antenna, even though it seemed to work well...

I figured out it was a base loaded 3/4 wave antenna!

I'm not sure what freq your antenna is designed for, but if I calculate the length of a 3/4 wavelength wire for 27 MHz, I get 26 feet.

If I take your 22.8 length and add the length of the coil you made (9 turns around 1.5 inch diameter = 42"), I get 26.33 feet...looks like a 3/4 wave antenna, but someone will point out that is not really true either.

HiDef also mentioned the the concept of their being an optimum length-to-diameter ratio for the coil, which I believe is also important. W8JI also mentioned this in one of his articles.

Inductors

Lots of other good reading there too...

Anyway, in my case I'm going to rebuild the classic feed as it was, maybe changing the materials a bit.

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Old 08-20-2009, 07:28 PM
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Quote:
Originally Posted by W5LZ View Post
And just like you predicted, somebody pointed that out as not being 'right'. But hey, everybody get's 'lucky' now and then!
Well, I had pretty good odds.

Quote:
Originally Posted by W5LZ View Post
So, 'mistakes' are to be expected.
And mistakes are often the cause of great discoveries...

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Old 08-20-2009, 07:37 PM
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Quote:
Originally Posted by Simon004 View Post
thanks for that bud, it gives me a good starting point. At the moment I have the main element I'm going to use, which is a shade under 22 feet in length, rather than change the bottom part of the half wave silver rod for a longer piece, I changed the top piece for half of the reflector from a 10M yagi that I've had sitting in my shed for a few years now, which gives me the length I need. I've used a piece of pine dowel to isolate the main element of the antenna from ground, and a 1.5 inch plastic tube (from one of my kids toys that they don't use) to wind the coil onto. The coil is at the moment about 15 turns, I found a piece of 16 guage antenna wire and wound it all on, with the intention of trimming it down untill I get a good match.

For the ground plane I'm using some more of the 10M yagi aluminium tubing, which is 4 foot in length, but I am also adding another 5 foot of copper wire and attaching it to the next radial like you did on your half wave vertical. I'm using 4 radials so the angle for the wire won't be as accute as it would be only using 3 ground plane legs.

I must admit I am a little confused now from the last post, allthough I understand that the matching coil on a 5/8th wave antenna does not replace the missing element length between a 5/8th and a 3/4 wave, the added inductance does simulate the antenna to give the same impedance match as a 3/4 wave would do.

So if you have a 5/8th wave element with a base loaded coil (no tap), it then becomes a 3/4 wave base loaded antenna, but if you have a 5/8th wave element with a tapped coil it is a 5/8th wave antenna? It might just be me, but that is how I read C2's post.
The point I was really trying to make is that it is a coincidence...

From what I've read (and interpreted) is that the matching device just transforms the impedance of the radiating element, whatever length that is...

As we unravel this onion, I continue to seek the real truths...namely, why are we even interested in the 5/8 wave design? Because it has some magical property?

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Old 08-20-2009, 10:55 PM
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What's the 'magic' about a 5/8w antenna?
It relates to how antennas radiate, the shape of that radiation pattern. If you start with a very short antenna in terms of wave length, the radiation pattern tends to wards being a 'ball'. The longer that antenna gets to be the 'larger' that ball begins to be, and it starts to sort of 'flatten' out a bit. Think of a balloon and pushing the top down towards the bottom sitting on a flat surface. The more you push down, the larger around the balloon gets. That 'hole' you've been pushing your finger down into gets 'wider' at the top and narrows at the tip of your finger. (And that's about as far as that analogy will go without some heavy duty imagination, which I'm not capable of describing all that well, so I won't even try.) Figure on that 'ball' turning into a sourt of 'saucer' shape at some point. That point is very close to about a 5/8 wave length. From there, when the antenna gets longer the 'flatness' of it's radiation pattern's shape starts getting less 'flat', gradually changing to some odd shape that's about the equivalent of that 'ball' again, as far as lower radiation angles go. that's a really crapy way of trying to describe it, sorry 'bout that. The point being that that 'change over point' is somewhere around a 5/8 wave length. that change over point is NOT exact by any means. There are just too many other factors that can affect a radiation pattern (it's environment, for instance).
If you want to consider 5/8w (.625w) that 'point', or 0.640w that point, suit yourself. There just won't be that much difference.
- 'Doc

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Old 08-21-2009, 08:32 AM
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So one thing I read is that 3/4 wave had a very high radiation angle, which is why I wanted to avoid it.

Maybe I'll just take some measurements on wire and coils...because it's fun.

I was finally able to get an Excel Smith Chart to work with the data my reflectometer spits out, so I can make graphs easier now...

Don't you just love Smith charts...?

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