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Portable 20 meter vertical




 
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Author Topic: Portable 20 meter vertical  (Read 1462 times)
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ka1tdq
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« on: November 12, 2017, 11:16:47 AM »

I wanted to build a portable 20 meter vertical that could break down easily, so I came up with this.  I'm about halfway done.

Everything will come apart by hand.  As seen in the picture, an 8' piece of 1/2" copper pipe will slide over the wooden dowel.  An 8' whip will be attached to the top of the copper pipe.  The wooden dowel isn't affixed and just slides out of the assembly.

Four wire radials will be attached to the copper plate via a wing nuts.  They'll be pulled taught on the ground by rope and small bricks.

I'll have a 1" tall square block that will slide over the wooden dowel to separate the copper pipe from the ground plane.  

The center conductor of the SO-239 will be attached to the copper pipe by a wing nut/bolt.

Jon


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AJ1G
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« Reply #1 on: November 12, 2017, 12:22:22 PM »

Looks like you are planning on mounting it fairly close to the ground.  You could also elevate to feed point to the top of a mast and use the radials and rope extensions of them to support the mast.  I've had my eye on the long telescoping fiberglas painting poles sold at Home Depot for portable antenna masts.
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Chris, AJ1G
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« Reply #2 on: November 12, 2017, 01:15:29 PM »

I was gonna say:  you'll be well served to get those radials a couple feet off the ground.

Well served by the dB gods, that is.

--Shane
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ka1tdq
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« Reply #3 on: November 12, 2017, 01:22:29 PM »

Yeah, why not? It’s just a threaded floor flange on the underside of the copper base. I just need to add an 8’ length of threaded pipe to the collection and screw that on too. The radials will serve as guy wires.

The plan was to get something a little more efficient than my MFJ resonator. And like always, my projects start out small and then get overkill. 😄

Jon


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WBear2GCR
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« Reply #4 on: November 18, 2017, 07:27:49 PM »

All things being equal, the copper plate seems excessive, unless you have a scrap pile full of copper plates.
I'd focus on a larger diameter for the vertical = more bandwidth...

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ka1tdq
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« Reply #5 on: November 18, 2017, 09:29:10 PM »

I've run a test with the vertical as-is and mounted on the ground.  It's actually very wide in bandwidth!  It starts out at 1.4:1 at 14.000 MHz and ends at 1.7:1 at the top of the band.  

I haven't posted a picture yet because I'm not finished with it.  I found out that the 1/2" wooden dowel bends too much with the vertical element on it.  I have a 48" fiberglass rod coming in Monday that should correct it.

Also, adding another vertical section would be too awkward to put together.  So, I put rubber non-skid on the bottom feet of the tripod, and the whole thing will sit on top of the car.  That'll give the radials a little extra height above ground.  

The car has an NMO-to-SO-239 adapter, so I'll have a short run of coax going from that adapter to the copper plate.  Electrically, the car will also be in the ground plane.  I'm hoping this has a positive effect.

Jon
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W6TOM
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« Reply #6 on: November 18, 2017, 10:10:45 PM »

  Using the car as a ground plane might just be what makes the difference. A few years ago I built a 20 meter dipole that I wanted to be easily movable, it would be in my backyard which I keep as bare dirt and just rotor till at the end of the rainy season, which is why I need to move it easily. I put 4 radials on it, tuned it for the center of the phone band, the driven element was 15' 1 1/2", match was excellent.

  It did not perform well at all, I was told I needed more radials.


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ka1tdq
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« Reply #7 on: November 27, 2017, 03:52:34 PM »

The fiberglass rod makes a big difference.  I first tried testing with the wooden dowel and it snapped in half.  

The SWR is unchanged in this configuration.  It takes about 15 minutes to rig up.  

Jon


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KD6VXI
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« Reply #8 on: November 27, 2017, 05:02:11 PM »

In general, as W6TOM saw, if your vertical has a super bandwidth, you have a lot of ground loss.

The radiation resistance becomes less and less a part of the feed point impedance, and the ground loss becomes more and more.

Ground loss R doesn't change from one end of 20 to the other.  A vertical element will.  If your swr is wide banded, you have too much ground loss.

Although yours doesn't seem all that bad, Jon.  Just something to keep in mind.

Get your radials horizontal.  Bet your bandwidth goes down.

--Shane
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KA2DZT
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« Reply #9 on: November 27, 2017, 07:18:36 PM »

Generally, radials sloping down raises the radiation resistance closer to 50 ohms.  Otherwise a 1/4 wave vertical with horizontal radials has a radiation resistance around 30 ohms.

20 meters probably could use more than four radials.  Four radials work OK at 10 meters and higher.  Lower frequencies need more.

Fred
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K1JJ
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« Reply #10 on: November 27, 2017, 09:01:30 PM »

Jon,

That's a very cool vertical for fixed-mobile!

As the guys said, more radials that cover a larger cross section of the ground will improve things. Just picture it as RF that needs a ground path to return to the antenna base. There is a certain "lamda"  or maximum spacing between ground radials before the losses become diminishing returns.


Here's a solution that may solve three problems... getting the radials horizontal, less noticeable to passers-by and more ground coverage /less loss. I assume the vertical's electrical base is connected to the car body and frame.

Connect 6-8  (16' long) radials to a large, copper battery-type alligator clip.  At the driver's side center connect the alligator clip to the car frame at the vehicle bottom. Spread out the radials on one side to cover 180 degrees. Then connect a second set of radials to the opposite passenger side car frame bottom and spread them out 180 degrees.   The 16 radials will all be on the ground doing a better job as well as easier to live with.  Stop by a fishing store and pick up some 2-3 oz lead sinkers to hold down each radial end - the ones that look like a tear drop with a hole in one end.

This vertical will absolutely perform as well as any full-size vertical home installation on 20M, (or better) especially if you are in the clear in a very large field with no obstructions or hills, or near the ocean  on wet sand, etc..

T

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KA2DZT
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« Reply #11 on: November 27, 2017, 09:10:01 PM »

"tear drop with a hole in one end" or more commonly known as a bank sinker.
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KA2DZT
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« Reply #12 on: November 27, 2017, 09:30:25 PM »

Years ago I made up a 25ft telescoping plastic mast from PVC.  Collapses down to about 5ft. for easy transport.  Held up with 4 guy ropes.  Used it with a 20M dipole.  Worked perfect, portable at a park or anywhere.  Had to hammer in four metal stakes to hold it up.

Probably a better idea than a vertical that requires a lot of radials.

I have a lot of portable aluminum masts to hold up dipoles, loops and beam antennas.  Highest reach 40 feet and the ones for beams have bases that can be rotated. Everything is custom made including the antennas.  Used mostly for FD in June.  They're designed to go up in minutes.  All held up with Manila rope.  Each mast has its own set of guy ropes attached to custom made guy rings.

The heavier masts with beams are pulled up using gin poles. Only takes one person to put up the antennas.

My stuff is no big deal compared to JJ's (the king of one man antenna installing) work.
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ka1tdq
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« Reply #13 on: November 28, 2017, 12:05:50 AM »

The goal was to build an antenna that's more efficient than the typical ham resonator, and could be erected easily.  This was a pretty good stab at it, but it still took a fair amount of effort to put it together.  Enough that I started to sweat...  Granted, I'm out of shape.  And this is just half the battle.  I also have to put together the cabling for my tablet/SDR receiver, QRP transmitter, coax switch, keyer, etc.  I've made it all neat, but it's still a chore.  By the time I call CQ, it could be a half hour later.

So, why did I do this again??

If nothing else to come from this, I found out that this electronic keyer kit from eBay is awesome!  I planned to use it in the car, but I paralleled it with my MFJ Econo Keyer so that I can have a memory message to call CQ.  It works great!  It's really nice just to hit a button and see the DX-60 and wattmeters come alive belching out 500 watts on 20 meters.  And I'm comfortable in my shack.  I can grab a soda and I don't sweat.

Jon


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