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Posted: 3/25/2017 2:47:01 PM EDT
Hi All
Trying my hand at making an antenna.

Concept: 2 pieces of 1/2 inch copper pipe fastened to a pvc connector for insulation.

Copper sleeve over both pieces at the center with feedline leads 1 and 2. Sleeves are so you can slide them out or in to adjust SWR.

No matter what I do my SWR is about 150 [meter is pegged]

Any advice guys? Shoot I made a 2 meter one out of a couple of pieces ow wire and it worked fine cant get this 2 meter one right for some reason

Thank you
Kid Couteau
Link Posted: 3/25/2017 3:26:19 PM EDT
[#1]
What are the dimensions you're starting with?
Are you feeding with ladder line or coax?
Link Posted: 3/25/2017 7:19:28 PM EDT
[#2]
Do you have a gamma match on it?
Link Posted: 3/25/2017 8:02:22 PM EDT
[#3]
Could you post a simple drawing? If so please list the dimensions on it.
How are you attaching the sliding sleeves to the elements?
Link Posted: 3/26/2017 12:18:13 AM EDT
[#4]
If I'm interpreting your post correctly, what you describe will not adjust the matching/SWR to any appreciable degree.

A photo might be helpful.
Link Posted: 3/26/2017 7:21:23 AM EDT
[#5]
A picture would help.

Right off the bat, how much of the pipe is inside the PVC?  That would be radiating as well, so if its long it would throw off your SWR.
Link Posted: 3/26/2017 7:53:02 AM EDT
[#6]
Pics






Oh and thank you all

I dont have the hose clamps holding the feed wires on the couplers in these pics but you can see how they would go on.
Link Posted: 3/26/2017 8:03:48 AM EDT
[#7]
Looks like you don't have a dipole, you have one continuous piece of pipe, which would be a monopole. You'll need to split the pipe within that PVC "T". Or is the copper pipe split within that white sleeve? It's hard to tell. Also, how much contact is there between the pipe and the sliding sleeves?
Link Posted: 3/26/2017 9:13:32 AM EDT
[#8]
Assuming that the Tee is plastic, and that it actually is a dipole separated by the center plastic coupling (and that your SWR meter is OK), then I gotta think that you have a wiring problem.  You should get some impedance reading, even if off a bit - so I'm guessing a short or open in the coax or connector.

When in doubt, ohm it out....
Link Posted: 3/26/2017 10:55:00 AM EDT
[#9]
Hi
The white sleeve is a pvc coupler and is just holding the two pipes in place but the pipes do not touch each other
Yes the T is plastic
Link Posted: 3/26/2017 11:15:40 AM EDT
[#10]
Depending on how close the two elements are inside the PVC it could be coupling. I did an adcock array in a similar.



Link Posted: 3/26/2017 11:20:01 AM EDT
[#11]
The antenna looks good, assuming, as you said, the two sides of the dipole are not touching.

Have you checked the coax and connector with an Ohm meter? It should read infinity from center pin to shell, and a resistance of a few ohms or less between center pin and center conductor, and between connector shell and shield.
Link Posted: 3/26/2017 3:57:46 PM EDT
[#12]
Moving those sliders back and forth is not going to change your resonant frequency. It'll change the impedance a bit at the same resonant frequency.

To really tune it you'll have to cut the copper pipe to the right length, really no way around that.

What length are the copper pipe sections now?
Link Posted: 3/26/2017 4:19:29 PM EDT
[#13]
I ohmed out the wire and it is good.
Less than 1 ohm on either part

The pipe was cut to 20 inches long
I am trying to make a 2M band antenna
Link Posted: 3/26/2017 4:59:12 PM EDT
[#14]
With each side of the antenna at 20 inches (total length = ~40 inches), the SWR should be under 3:1. Because your antenna is made of tubing, the added capacitance will cause it to resonate at a shorter length than if it was made of wire. The bandwidth will be wider, however, which is a plus. The difference will not be that much.

Try measuring the SWR at the low and high ends of the band. If the low end SWR is lower than the high end, the antenna is too long. If its higher, the antenna is too short.

Here's how to trim an antenna's length to get it acceptably close to resonance: http://www.hamradioschool.com/trimming-your-dipole-antenna/

While we're discussing antennas, here's an article on antennas, feedlines, SWR, and propagation. It's YUUUGE, so you might want to print it out and keep it around for reference. http://www.hamuniverse.com/n4jaantennabook.html
Link Posted: 3/26/2017 8:25:36 PM EDT
[#15]
Assuming you cut the antenna to a correct length and you don't see any resonance on or near the 2m band:

What are you measuring SWR with? I assume you are using some kind of antenna analyzer, right?
Most likely you feed it with a 50 Ohm coax (unballanced). How is coax routed in relation to the antenna?

What happens if you put the antenna analyzer down (not holding it in your hand when doing measurements)? If SWR changes, put a few snap-on RF chokes on the coax, as close to the antenna's feed-point as possible. Your coax may be radiating and throwing off the measurements. It's worth looking into, before you start making any major changes or modifications.
Link Posted: 3/26/2017 10:58:51 PM EDT
[#16]
Thicker elements will be much shorter at the same resonant frequency than thin wire. Off the top of my head you're probably at least an inch too long.

If you have an analyzer, this would be relatively trivial to find the resonant frequency and see how much you need to trim.
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