Bizarre SWR shift

[W9WD]

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

I took the coil loose from the bottom of the vertical element.
Lowered the ground radials plate
Then removed the vertical element. Then slid the ground radials up off the mast
While it was down I put the piece of 7/8" up inside the vertical and then put the 10-32 screw back.
Reversed procedure to install

 

[W9WD]

Tried jumping the ground plane to the mast and it didn't change anything.
Took out the 7/8" bar and it changed a lot. I stuck my shorting jumper in the same spot as it was and I got 2.5:1 on 27.405.....bizarre

I'm using a crappy CB to get in the SWR ball park so I don't goof up my good radio so my target frequency of 27.9 is still to be discovered.

I'll try working around and round the coil and see if I can get it any closer to what it was before I moved the antenna.

I am encouraged

 

[W9WD]

Quite by accident, I discovered that if I grip the mast below the ground plane and slide my hand up or down I can change the SWR. The closer I get to the bottom of the ground plane the lower the SWR. If I touch the ground plane the SWR drops to 1:1.

However....
If I take a jumper from the ground plane to the mast the SWR jumps way up

 

[W5LZ]

Got a feeling that gray PVC -may- have something to do with it. But did you use the same stuff on the short post?
Another thought is that you standing next to it, and running your hand close to that groundplane, you might be introducing a little more capacitive reactance into things. If so, and that's a bit 'if', a little less coil may be the answer.
- 'Doc


(Thats one reason I really like using one'a them analyzers, it gives a better idea of what's going on without quite as many 'if's.)

 

[W9WD]

Got a feeling that gray PVC -may- have something to do with it. But did you use the same stuff on the short post?
Another thought is that you standing next to it, and running your hand close to that groundplane, you might be introducing a little more capacitive reactance into things. If so, and that's a bit 'if', a little less coil may be the answer.
- 'Doc


(Thats one reason I really like using one'a them analyzers, it gives a better idea of what's going on without quite as many 'if's.)

Yes it's the same gray pvc. It's exactly the same as it was on the short pole just a longer pole.

I took the insulator out from between the ground plane and the mast and mounted the ground plane straight to the mast and that was no good.

I just got back from Home Depot where I bought a nice big grounding rod and pounded that in.
I'll hook that up and see what that does.

(Wish I had one of them analyzers)

 

[Moleculo]

That coil wire doesn't happen to have coating of any kind on it does it?

 

[W9WD]

That coil wire doesn't happen to have coating of any kind on it does it?

I don't believe so.
It's just Home Depot Ground wire. I am using an alligator clip on the end on my shorting strap and I am wiggling it's teeth back and forth as I set it.

Late breaking news....

Grounding the mast with a grounding rod seems to have helped immensely. I am looking at SWR(s) on the CB channels below 1.5:1

I am routing the coax now and will see what I can get on 27.9 from the table on the ground.

Boy the skip was rolling today.

 

[W9WD]

I hooked up the 2950 and I am getting 1:1 at 30mhz.
Soon as the wife gets home we can get it tuned up

 

[W9WD]

This thing really has me going.
If I turn the power down I can get flat SWR form 11 to 10 meters. not good but flat 1.9:1
When I increase to the power (to 10 watts) the SWR goes up to 3:1

I'm about ready to try something else.
Maybe toss the coil and try a ring tuner like the V58 ?

 

[HomerBB]

I can not say that I fully understand your problem, or that I fully understand antenna theory. Some things I did on mine was to keep the potentially conductive metal vertical and other metal parts - coil, ground plane radials - apart; isolated both by insulators, and alignment to each other. I kept the vertical above the coil bridging the GP and the vertical with the opposite ends of the coil. The coax shield was directly connected to the ground plane, and the coax center conductor was tapped into the coil for best SWR. Perhaps the thickness of the vertical played into the match (don't know, just mulling). It is 1 1/4" at the base tapering to 1/2" at the tip. Also, as I was led to believe, I set the ground plane at 45* angle downward from the horizontal in order to help me achieve match easier. I wanted to keep them horizontal for a superior radiation angle, but desired a 50ohm match at the feed point more.

I wanted to toss mine, too, especially when it kept blowing down and SWR moved around on me, but I didn't.

Hope this helps.

As we say at the Home Depot (thanks for shopping with us), "You can do it, we can help"

 

[W9WD]

Yes I think you are right on separating things.
I attack again today

Never give up! Never Surrender!

 

[HomerBB]

Yes I think you are right on separating things.
I attack again today

Never give up! Never Surrender!

The closet rod dowel I used at first broke when I put it up into the vertical element.

I switched to this: (not to scale)

dowel = black

1 1/4" PVC = blue

1 1/2" PVC = gray

vertical element = red

coils = green

the dowel slid into the 1 1/4" PVC

the 1 1/4 PVC slid up into the 1 1/2" PVC, but not all the way to the top end (required a slit along the 1 1/2" PVC length)

the vertical slid down into the upper end of the 1 1/2" PVC bottom end of it resting on the top end of the dowel.

I wrapped my coil around the portion of this arrangement where the dowel,
1 1/4", and 1 1/2" sections all overlapped for triple layered integrity.

(the upper end of the coil attached to the bottom of the vertical.

the lower end of the coil to the GP radials along with the coax shield.

the center conductor to the tap point on the coil for best SWR.)

You can see all the electrical components are not overlapping

BTW, GP radials attached below the coil onto the 1 1/4" PVC pipe via the 1 1/4" x 4" PVC adapter I used. Crude, but effective.

 

[W9WD]

Was that the fiberglass dowel that broke or wood?
What diameter?

 

[HomerBB]

Was that the fiberglass dowel that broke or wood?
What diameter?

I used wood first, the I poured fiberglass into a section of 1 1/4" pvc and it broke. I am convinced a well made fiberglass rod would probably work. I also tried to use the vinyl closet rod to insulate my A99 from the metal mast when I added the GP kit. It broke.

 

[Marconi]

Yes I think you are right on separating things.
I attack again today

Never give up! Never Surrender!

There you go 8324. Hang in there.

IMO, when I see, as you say regarding SWR, "...not good but flat," the antenna system's complex impedance is likely showing a lot of losses, besides ohmic losses (increased reactance), and very little radiation resistance. This condition may combine to show a good SWR, but with very low efficiency. As 'Doc likes to say and is correct in saying it, "...reactance doesn't radiate."

The combination of reactance, resistance, and ohmic losses make up this complex impedance at the feed point. The result, good or bad, may even produce a decent SWR, but depending on the ratios---the antenna may not work very well. So when our systems shows a lot of reactance we may be resonant somewhere, but usually not where we wish or plan.

Like Homer suggests, fix (separate) the potential problem of the radiator next to the support mast and I think you will be fine. You may need to redesign and raise your coil and the GP up closer to the radiator in order to reduce the added inductance that you likely show at present. I think you may only need a few inches of exposed insulator in the top of your mast to get the radiator up high enough without adding a lot more inductive reactance and your ground plane needs to be as close as possible to the feed point.

Since you said, at some point, that your antenna showed you a perfect match, high up in the 30 meg range, I may be all wrong in my thinking. If not, then you should have been resonant down lower in frequency not higher.

The forumlas for calculating antenna length depend on a few factors that I think you understand, but a 5/8 wave antenna first off is likely to show a low frequency---if the matching device is inductive with no capacitance at the base of the antenna, or the radiator is large in diameter and very long.

Good luck,