Bizarre SWR shift

[W9WD]

I built this 5/8 wave and have been testing it on a short pole about 8' off the ground





I was getting SWRs of

1.3:1 at 28.5
1:1 at 27.9
1.4:1 at 27.325
1.5:1 at 26.965

I used a coil tuning system like this




I was satisfied with it's SWR performance, so today I moved it to the top of a pole attached to the house at about 23'.





The only thing I changed was it's height/location and I slid a 12" piece of 7/8" round stock up inside the 1" lower section of the vertical element so it would not be crushed when I tightened down on the mounting U-bolts.

Now I can't get the SWR below 1:3 and no matter where I try to set the jumper it gets worse. The 1:3 is when the jumper is placed where coil attaches to the vertical element.

Any suggestions?

 

[W5LZ]

I can understand why you would want that SWR lower, but there's nothing wrong with a 1.3:1.
I'm curious, are you adjusting that shorting strap 'around' those coil turns, or just straight up/down? The straight up/down is a 'coarse' adjustment, the 'around' thingy is a fine adjustment.
That insert to keep the tubing from being crushed isn't going to make any difference if it isn't making a contact it shouldn't make, I sort of doubt if it is.
The biggest difference when moving an antenna is that you are changing it's reactance with what's around it. Height means that's with the dirt under it. That is normal for any antenna, sometimes it makes a big difference, sometimes not (figure on it making a difference). You can use that, it isn't always a detriment! One way is by the 'angle of the dangle' of those radials. Seeing how they are attached, that may not be an easy thing to manage, or even wanting to do it. It certainly isn't a sure 'cure', so think about it before doing it. If you're not sure of getting it back like it was, I think I'd pass on that option. If they aren't already, try shorting them to the mounting post. Might work, can't hurt.
So far it sounds like things are about 'normal'. What would really be bizarre is if it didn't change!
- 'Doc


(There's a lot of things in here that I figure you already know, it's just that I don't know if you do or not. I'm not trying to be insulting, I do insults a lot better than that!)

 

[W9WD]

What I meant to say was 3 to 1
I am moving the shorting strap around the coil, but the SWR gets worse the further I go away from the vertical element. I can get it down to 3 to 1 by hooking the strap directly to the base of the vertical element.

When I initial did my adjustments on the short pole I moved the grounding strap up and down and round and round until I finally got the sweet spot. I imagined that I could just transfer the antenna to the taller pole and get the same SWR

When I moved the antenna I was very careful to not move anything and all I did was undo the coil from the bottom of the vertical element and move the whole thing up to the taller pole.

I was really shocked (surprised) when I got the bad SWR readings

 

[Moleculo]

What is that tubing made of that is in between the vertical element and the radials?

 

[HomerBB]

I'm wondering if you are measuring with all that meatal scaffolding around it. I'd think if you are, then you've given it a very different counterpoise than it had...

just thinking out loud. hopefully you'll get it back the way you want it.

 

[W9WD]

What is that tubing made of that is in between the vertical element and the radials?

You mean the coil part?
It's #12 copper (ground wire)
10 turns 2-3/4" long, 1-1/2" in diameter

The mast is 1-1/2" sched 40 pipe (1.900" OD)
The vertical element and the radials are both isolated from the mast using gray pvc electrical pipe

 

[W9WD]

I'm wondering if you are measuring with all that meatal scaffolding around it. I'd think if you are, then you've given it a very different counterpoise than it had...

just thinking out loud. hopefully you'll get it back the way you want it.

I pretty much have to stand on my toes to get up to the vertical element.

I took the coil and connector off and took it inside and check continuity all over and nothing is shorted. I'll put it back up and check again and if I still get wacky readings I'll take off the vertical element and pull that solid piece of 7/8" aluminum out of it and see what that does.

Tonight I take my Technician test so I'm a little distracted.

 

[Marconi]

8324, in image 58_7 below I see the ground radials are insulated from the mast. Is the GP mounting plate conductive? How is this mounting plate secured to the PVC and the mast?

I also understand from others that gray pvc has high metal content and may be conductive.

I see the base of the coil is directly attached to the L-bracket supporting the feed point, but how does this bracket attach to the ground radials in the image above?

It may be OK to insulate the antenna from the from the grounded mast, but doesn't the base of this coil need to connect directly to the counterpoise radials somehow?

When you were testing very low to the earth, your loss may have been so high (with a good earth return) that you are able to see a reasonable match on your inline meter, but when you raise it up you start to loose this earth advantage and that is when the counterpoise is supposed to start to work providing a return path. I suspect that your radials are not attached to the anything but the mounting plate.

I also question having the radiator so close to the grounded mast regardless of whether the radiator is insulated from the mast or not. It looks like to me that your radiation is starting way down by the feed point and you are getting reflection off of this mast for a foot or more.

I could be wrong, but I see problems. Maybe I have not explained myself well or maybe I misunderstand what I see, but this is my opinion. Don't let me discourage you though, I like and encourage your efforts

 

[W9WD]

8324, in image 58_7 below I see the ground radials are insulated from the mast. Is the GP mounting plate conductive? How is this mounting plate secured to the PVC and the mast?

Yes it is conductive it's a piece of 1/2" aluminum plate. It is held by two set screws in the side that push against the pvc

I see the base of the coil is directly attached to the L-bracket supporting the feed point, but how does this bracket attach to the ground radials in the image above?

The "L" bracket is attached to the aluminum plate and so are the radials

I suspect that your radials are not attached to the anything but the mounting plate.

The ground of the coax hooks to the "L" bracket and the "L" bracket hooks to the mounting plate and the mounting plate hooks to the radials. One end of the coil hooks to the "L" bracket and the other end hooks to the vertical element. The center of the coax hooks to a jumper along the coil.

I see this thread was moved to the CB section, but the antenna was cut (22') to be in between 11m and 10m so I could use it both for CB and Ham

I think first thing tomorrow I will pull out the 7/8 bar and see what happens

 

[W5LZ]

It has really been one of those days.
Had it all thought out, just knew I'd found the problem, even drew a picture. Posted it, looked at the picture... Crap, same as what's there to start with. Never mind, I'm surprized I haven't peed on myself today...
- 'Doc

 

[Marconi]

Thanks for the reply and sorry if I hijacked your thread a little to show you a point. Explains everything but the potential problem for the mast beside the radiator. Example of a similar problem mount I had with a homemade 1/4 wave antenna not allowing me to tune the antenna even though it was a small infraction.

When I tried to use this setup and keep the ground plane close to the feed point, the approximate 4" of swaged lenght on the end of the support mast that you see next to the base receiver for the 1/4 wave whip was too long by about an inch and prevented a tune with the antenna. See the simple solution in the bottom image where I cut off about an inch. This allowed me to get an immediate tune with just a simple cut using a hacksaw.

 

[Moleculo]

If I'm looking at the pictures correctly, the problem is that the mast that the driven vertical element is attached to is metal which is also connected to the groundplane. That section should be non-conductive...like a piece of heavy fiberglass rod. Then you use the matching coils between the driven vertical elent and the groundplane radials. The radials can be attached directly to the mast. In your picture, I don't see any insulation in between the vertical element and the radials (matching coil not included). Look at this picture to see what I'm referring to. Even though this antenna uses a type of Beta match, it's the same principle: Double Trombone Impedance Matching System

BTW, I moved this thread to the CB Antennas section thinking that was the intended application, but it can certainly be moved back if it makes a difference to you.

 

[Marconi]

If I'm looking at the pictures correctly, the problem is that the mast that the driven vertical element is attached to is metal which is also connected to the groundplane. That section should be non-conductive...like a piece of heavy fiberglass rod. Then you use the matching coils between the driven vertical elent and the groundplane radials. The radials can be attached directly to the mast. In your picture, I don't see any insulation in between the vertical element and the radials (matching coil not included). Look at this picture to see what I'm referring to. Even though this antenna uses a type of Beta match, it's the same principle: Double Trombone Impedance Matching System

BTW, I moved this thread to the CB Antennas section thinking that was the intended application, but it can certainly be moved back if it makes a difference to you.

Moleculo, you're right, except the vertical element looks to be insulated with small lengths of gray PVC under each u-bolt with the radiator passing through.

I think that the mast, setting along side of the radiator is a no-no in construction. This is probably why his tap point is so high on the inductor coil too.

 

[W9WD]

If grounding the radials is the issue it seems like I could just take an alligator clip jumper and connect the radials to the mast at the top of the pipe, temporarily. I should see a difference on the meter if it does have an effect.
I was also thinking that I could ground the braid of the coax to a water pipe with a jumper (at ground level), but that would make the coax part of the antenna ground system and might make things worse ???

If grounding the radials produces results it will be pretty easy to just drill a hole in the gray pvc that is currently isolating the radials and let one of the set screws make contact with the mast.

 

[W5LZ]

When you moved the antenna from the short pole to the longer one, how did you do that? I'm wondering because of the way the vertical element and the radials are not 'connected' except for the coil. Did you take something loose and then re-attach it?
- 'Doc