bit of an extreme design: co phased vertical with base load

[Nationalpatriot]

Question-- why use coax for the verticals? A piece of wire would do it-- cheaper, lighter, no capacitance or velocity factor (practically) to worry with. If you wanted self-supporting, I'd probably go with 1/2" EMT in this case.

Right now just because I have extra coax, what would 2 pieces of 1/2" emt about 10' long cost right about now? (doubt I could get it in 9 foot pieces haha)
(also how would I use it if I could fine some cheap EMT? just drill a small hole and solder the radials into the hole?)

 

[RickC.]

I think 1/2 EMT sells for less than 3 bucks for a 10 foot piece, just cut it with a hacksaw, etc.

The EMT would have to be insulated from the ground- the center conductor of the coax attaches to that, the braid to the radials.

I really do like the idea of you doing this project, but seriously, read up on verticals before you build this. It's not complicated and I'd like to see you build this array, but some more basic knowlege would help you here.


Rick

 

[Marconi]

Question-- why use coax for the verticals? A piece of wire would do it-- cheaper, lighter, no capacitance or velocity factor (practically) to worry with. If you wanted self-supporting, I'd probably go with 1/2" EMT in this case.

RickC and Nationalpatriot, IMO there are many advantages with 11 meters, being able to raise the antenna and the ground plane up high, and improved performance is probably the best of the advantages as you get above ground clutter. This is not always the case with the lower frequency antennas unless they are very short and inefficient, so ground mounting is almost a necessity in such cases. Why even mess with all the problems associated with ground mounted antennas that has a radiator that is only about 9' tall?

Using coax to make an antenna is fine and it can produce a better bandwidth than a skinny wire, but the problem is support, just like with any wire.

RickC, considerations for capacitance and velocity factors are only important regarding coax use when the current flows inside the coax, and not for current that flows on the outside---like NP suggest in his project. An antenna made with coax is just wire, and the shield is the part that your RF will flow on. I would solider the shield and the center together to add some support to the wire however. Otherwise using coax is just like wire, except in this application it is actually lighter than a comparable thickness of plain wire, and producing a wire cross section for the RF that is maybe 10 times that of a small diameter wire. This is where the added bandwidth may come from.

I think 1/2 EMT sells for less than 3 bucks for a 10 foot piece, just cut it with a hacksaw, etc.

The EMT would have to be insulated from the ground- the center conductor of the coax attaches to that, the braid to the radials.

I really do like the idea of you doing this project, but seriously, read up on verticals before you build this. It's not complicated and I'd like to see you build this array, but some more basic knowlege would help you here.

Rick

OK Rick, that is some good advice, considering NP wants to build an array which will be much easier than raising it up. At 11 meters however, I don't think NP could produce a ground mounted array that will perform better than even a 1/4 wave ground plane raised up 30' to 40'. Only exception might be, unless he is really out in the wide open spaces away from his house etc,. and has a radio shack out there too to cut down of feed line length.

 

[RickC.]

No argument from me on that, but if there's an area in the clear it won't hurt (or cost much) to try it. Who knows, we may have another Bill Orr (I almost said Marconi, but um...) in the making here.


Agreed about the coax up to a point. The experiments I did with coax for a 30 meter ground plane some years back (don't ask, just thought I'd try it) ended up with the finished vertical element being about 5% shorter than the calculated length. Since it was a quick and dirty antenna that I have not tried to duplicate since I can't give any more info that than-- but the foam dielectric apparently did have an effect-- not what I expected to see, but it was what it was.


Just trying to save our man here a little grief on what I think is an admirable project.


Rick

 

[Nationalpatriot]

I think 1/2 EMT sells for less than 3 bucks for a 10 foot piece, just cut it with a hacksaw, etc.

The EMT would have to be insulated from the ground- the center conductor of the coax attaches to that, the braid to the radials.

I really do like the idea of you doing this project, but seriously, read up on verticals before you build this. It's not complicated and I'd like to see you build this array, but some more basic knowlege would help you here.


Rick

I am at work right now and cannot search google, any reputable places to research verticals? (I have searched literally hundreds of websites, I think I've filled my mind with too much information and no filing system)

 

[Nationalpatriot]

RickC and Nationalpatriot, IMO there are many advantages with 11 meters, being able to raise the antenna and the ground plane up high, and improved performance is probably the best of the advantages as you get above ground clutter. This is not always the case with the lower frequency antennas unless they are very short and inefficient, so ground mounting is almost a necessity in such cases. Why even mess with all the problems associated with ground mounted antennas that has a radiator that is only about 9' tall?

Using coax to make an antenna is fine and it can produce a better bandwidth than a skinny wire, but the problem is support, just like with any wire.

RickC, considerations for capacitance and velocity factors are only important regarding coax use when the current flows inside the coax, and not for current that flows on the outside---like NP suggest in his project. An antenna made with coax is just wire, and the shield is the part that your RF will flow on. I would solider the shield and the center together to add some support to the wire however. Otherwise using coax is just like wire, except in this application it is actually lighter than a comparable thickness of plain wire, and producing a wire cross section for the RF that is maybe 10 times that of a small diameter wire. This is where the added bandwidth may come from.



OK Rick, that is some good advice, considering NP wants to build an array which will be much easier than raising it up. At 11 meters however, I don't think NP could produce a ground mounted array that will perform better than even a 1/4 wave ground plane raised up 30' to 40'. Only exception might be, unless he is really out in the wide open spaces away from his house etc,. and has a radio shack out there too to cut down of feed line length.

I'm in a wide open space, but have county restrictions on antenna height (don't ask me why, maybe crop dusters?) the antenna I have now really doesn't work badly, I just want to experiment. (and I do have a radioshack nearby that supplies me with coax happily, it's just my wallet that can't always supply THEM with the money haha)


EDIT:
Also, does the EMT have to be a different height than normal? if not, what EXACTLY does the length of it have to be? (I am seriously considering this idea, it saves on PVC which is a B*itch to buy anymore and is more resistant to weather in my opinion.

 

[Marconi]

No argument from me on that, but if there's an area in the clear it won't hurt (or cost much) to try it. Who knows, we may have another Bill Orr (I almost said Marconi, but um...) in the making here.


Agreed about the coax up to a point. The experiments I did with coax for a 30 meter ground plane some years back (don't ask, just thought I'd try it) ended up with the finished vertical element being about 5% shorter than the calculated length. Since it was a quick and dirty antenna that I have not tried to duplicate since I can't give any more info that than-- but the foam dielectric apparently did have an effect-- not what I expected to see, but it was what it was.


Just trying to save our man here a little grief on what I think is an admirable project.


Rick

That's OK Rick, Marconi is just my handle. The length error you made in your calculation, whatever it was, had nothing to do with the VF or the dielectric of the coax used as a radiator on your antenna.

Why did you build a 30 meter vertical ground plane out of coax anyway?

Or, am I misunderstanding what you are saying here?

 

[RickC.]

Ha, well, we'll agree to disagree on that-- I think it ended up being just under 22' when it was said and done.

Only the vertical radiator was coax, and as I recall I just got a wild hair and decided to do it since I had a length that was just over 23' long. It worked fine, but I got tired of 30M and homebrew QRP (xtal with warp) pretty quickly!

 

[W5LZ]

Conductor diameter and signal bandwidth. At HF, it's just not something to worry about, or expect much from, until that diameter get's really big. How big at 11 meters? Something on the order of several inches in diameter. Fractions of an inch just aren't going to make any significant differences in the antenna's bandwidth.
Velocity factor.
If you are using coax as the conductor making up an antenna, forget it, it doesn't come into play at all. Neither does that dielectric insulation between the braid and center conductor, it's invisible to RF, won't change anything or make any difference.
The place to worry about that velocity factor is with the 75 ohm phasing harness, it definitely comes into play there! You have to use it to find the correct length(s) for that phasing harness. Typical 75 ohm coax has a velocity factor of .66 or 66%. So for 27.185 Mhz (ch-19) a 1/2 wave of 75 ohm coax would be about 11.9 feet. Call it 12 feet. So, an electrical 1/4 wave length of 75 ohm coax for ch-19 would be just a tad under 6 feet. To get those two antennas 9 feet apart, you'd have to use 3 electrical 1/4 wave lengths from each antenna to the 'T', so roughly 36 feet of 75 ohm coax total. Odd multiples of a 1/4 wave means you still have the equivalent of a 1/4 wave at the end of that odd multiple. Even multiples means you wind up with a 1/2 wave, and that won't work for phasing antennas. (That's a 'given', not just an 'opinion'.)
Ain't all this stuff fun??
- 'Doc

 

[Nationalpatriot]

alrighty boys I picked up the 1/2" EMT on the way home and found some 75ohm rg-6 to use. Can ya'll give me some instructions?

-what length exactly should the harness be
-what height should the poles be (my primary channel is 20)
-Do I absolutely need a T from the harness to the line to the CB radio? or can I solder it and heat shrink tube it shut


Thanks!

 

[RickC.]

NB,

Have a look at this-- skip down to the addendum-- "Amateur Phasing" and see figures 1 and 2. There is a misprint there-- that should be 2.3 dB gain, not 23!

http://www.hy-gain.com/man/pdf/AV-12AVQ.pdf

 

[Nationalpatriot]

NB,

Have a look at this-- skip down to the addendum-- "Amateur Phasing" and see figures 1 and 2. There is a misprint there-- that should be 2.3 dB gain, not 23!

http://www.hy-gain.com/man/pdf/AV-12AVQ.pdf

Ahh very good information, thanks Rick! That shows me a lot of what I needed to know. I have to visit my girlfriend's new baby niece this evening but I'll start first thing when I get home and post some pictures of the construction process. Anyone have any sophisticated ideas to test the range of this contraption?

 

[W5LZ]

Can you do away with that 'T' connector and just connect the two phasing cables and the 50 ohm feed line (if any) together? Yes, you can, just be careful doing it, then insulate it good, and even better, put that connection inside out of the weather. That typically can result in an impedance 'bump', but it doesn't have to be all that bad.
Because of the inconsistency of manufacturing -any- coax cable the 'numbers' don't have to be all that 'exact'. Using 27.200 Mhz for the frequency will get you as close as anything will (basically the center of the band). The usual formula results in feet. Multiplying the answer by 12 will give you inches, and that's about as close as you need for HF. You'll end up with a 'ball-park' phasing line.
Tune the two antennas FIRST, then leave them alone. Any 'adjusting' after that can be done with the phasing lines. That's not simple, or easy, but is better than having antennas that are not identically tuned. Using something like an MFJ-259 makes the whole thing much easier, but also much more expensive (unless you can borrow one?).
(It's easy for someone else to spend YOUR money, ain't it?)
Have fun.
- 'Doc

 

[CDX-007]

I'm surprised no one mentioned the cancellation and directionality of the ½ wave spaced 'array'.

Maybe you already know this, but placing two antennas ½ wave apart puts them 180° out of phase so when you draw a line running from one to the other antenna you will have cancellation along the projection of that line in both directions, ie: N-S.
Perpendicular to that line, or 'broadside' to the antennas (E-W) you should have gain over only one antenna. This is called a figure-8 or cardioid pattern

If you wanted better omni-directional performance than just a single omni antenna, simply place them 9' apart, not 18'.

The length of the ¼ wave antennas, using ½" EMT should be right about 8' 6" for center-of-band 27.185mHz.

You need to find out the exact velocity factor of the RG-6 from the manufacturer then multiply it by 108" to determine the exact working length of each ½ of the 75ohm phasing harness for ¼ wave (9') spacing. For ½ wave spacing they will both need to be 3/4 wave long to reach the T connector.

I would strongly recommend using a T connector and SO-239 connectors with appropriate down sizing spacers for the RG-6.

If you cut a piece of RG-6 1¼ wave long for one side only you will effectively turn your array sideways and the cancellation will then be broadside to the antennas if they are 18' apart.

If you wanted to set it up for switchable 90° E-W or N-S performance it would require two SPDT relays and a DC relay switching voltage wire to select between the 1¼ and 3/4 wave pieces of RG-6 on one side only.

If you elevate the antennas so they are just high enough to allow a 30° down slope of ¼ wave long elevated radials for each antenna, (maybe 3' antenna base height) you will only require 4 radials each, and should perceive gain over a flat ground mounted system, plus your SWR will be closer to 1:1 as each antenna will be ~52ohms instead of ~37ohms.

&&&&&&&&&&&&&&&&&&&&&&

- Later you could add 2 more antennas, all 9' or ¼ wave apart in a square configuration, and build a switchable phasing harness for a "Four Square" antenna array and WOW, what a superb selectable directional!

Our 4 square for 80m at the cabin is simply amazing. Provides strong performance with signals which just aren't there on the wire or the Butternut vertical.

 

[CDX-007]

It looks/sounds like you meant to post a graphic, I wish you had so what you described made more sense to me.
It sounds as if you plan to use some excess coax for the antenna? That can work, but it has disadvantages with it.
Ground mounted antennas also work. They usually require much more than just 3 or 4 radials, or just a ground rod or two. They can certainly be more practical for your situation, so it seems.
You're right about the phasing/timing coax being 75 ohms, or even high impedance if you can find it. While you're at it, you might as well double the amount of that 75 ohm coax and get an antenna switch to switch the direction of the radiation pattern. It's not that difficult, especially with ground mounted verticals. Beats only being 'pointed' in one direction.
Lots of possibilities.
- 'Doc

For clarification:
Coax vertical antennas are a problem cuz it's hard to make them stand up all by themselves. There's no real advantage to them unless you just have a lot more coax than you'll ever need. As long as it's conductive, you can make an antenna out of anything ... almost, wet noodles are a real PITA to make work.

Phased antennas are not beams, they are bi-directional, N to S for instance. If you switch that phasing 90 degrees, they become E to W. The radiation patterns for phased antennas are not 'razor' sharp by any means. More a 'wide angle lens' pointed in two directions forwards/backwards.

Ground mounted antennas work just dandy. Broadcast stations have been using them for a very long time.

Well it appears W5LZ already did. - I just found it.