Me tuning vertical = Headache

[AgentCOPP1]

So I've been an HF ham (General) for about a month now, and just recently I bought myself an SWR meter. I had absolutely no clue what my SWR would be, but when I turned it on and transmitted...

Wow. 4:1 it was reading! No wonder I couldn't make any DX contacts. So I got out and tried for the first time to tune my vertical with the help of some hams on the local repeater. After trial and error, I finally got the antenna to be resonant on 20 meters with an SWR of 1.5. I got this after about an hour of reading the meter, unhooking all of the radials and coax, taking the antenna off it's mount, laying it on the ground, adjusting, putting it back on the mount, reconnecting the radials and coax... it's a lot of work.

Actually, I figured out that the metal pipe that I was using to put the antenna in the ground was causing an absurd amount of abnormalities in the SWR readings. So I took the pipe out of the ground and just stuck the base of the antenna into the hole. This fixed a lot of problems.

Even with the antenna's length matching exactly the specifications on the manual, it was resonant about 1 MHz below the 20 meter band (which is all I was worrying about at the moment). But the weird thing is that it was EXTREMELY resonant on the 40 meter band. We're talking 1.1 with the radio putting a strain on my power supply (which is a good thing). Eventually I got it around 1.5 in the middle of the 20 meter band, but now it's 4:1 on the 40 meter band!!!

Why does it have to be this annoying? Can you please give me some advice, because I'm really frustrated with this

Also, when I was trying to tune the antenna on the 20 meter section of it, changing the length of this section did hardly anything. The 20 meter portion is the last segment of it, and no matter how much I changed its length, the SWR would hardly budge. I made it as short as I possibly could before I had to start shortening the 10 meter portion on the bottom of the antenna. That should not happen. All of the lengths were exactly where they were supposed to be, and based on my calculations, a 20 meter quarter wave antenna should be roughly 16.4 feet, but my antenna is resonant at 20 meters with only about 12 feet.

Antenna is a Cushcraft AV-4 quad bander vertical antenna. Its longest portion is lengthened for 20 meters, but it has a capacitance hat which is (I'm assuming) to make the antenna resonant on 40 meters as well.

Any ideas would be greatly appreciated.

Edit: Just for clarification, when I got the 20 meter SWR down to 1.5, the antenna became resonant at roughly 500KHz above the 40 meter band which is expected. But how do I keep the 20 meter SWR the same while making 40 meters lower?

 

[popnokick]

Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 6_1_4 like Mac OS X) AppleWebKit/536.26 (KHTML, like Gecko) Version/6.0 Mobile/10B350 Safari/8536.25)

Hey.... Work 40 and be happy!

 

[N5TWB]

Are you using any ground radials as part of your antenna system?

I can't even find a review of an AV-4 on eHam but there is one for an AV-3 that says it was a challenge to tune so it seems maybe you shouldn't feel alone on this.

 

[AgentCOPP1]

Are you using any ground radials as part of your antenna system?

I can't even find a review of an AV-4 on eHam but there is one for an AV-3 that says it was a challenge to tune so it seems maybe you shouldn't feel alone on this.

Yes, I'm using ground radials. I've got 6 buried 20 meter radials and 3 buried 40 meter radials. I've got 6 for 20 mainly because I want 20 meters to perform the best out of all the bands.

 

[prcguy]

Tuning should be fairly easy if you understand all the parts that make up the antenna. Its a 4 band trap antenna and the element length below the lowest trap determines the 10m resonance, the next trap up is for 15m, the next one up is for 20m and adjusting the top element will affect 40m.

Since anything above a particular trap will usually be affected when adjusted you would tune 10m element first then work up the rest of the bands and the short section below the 20m trap should not affect 15m or 10m and the 40m top adjustment should only affect 40m.

On the SWR meter, if an element is too short the match will be better at the higher end of the band and if its too long the match will be better at the lower end assuming your not too far off when you start.
prcguy

 

[nanZor]

One thing to keep in mind is that SWR does NOT always indicate resonance.

For example, on a perfect quarter wave vertical, the impedance should be 35 ohms resistive, not 50 ohms. This would actually be 1.5:1 swr. Tuning it to 1:1 swr is actually tuning *away* from being the most efficient generally speaking.

I'd also suggest a 1:1 balun / rf-choke at or near the vertical's feedpoint.

 

[prcguy]

35 ohms would be with a perfect ground under the vertical and the OPs description of the ground is not so perfect. The feedpoint impedance will be raised on a vertical when the ground is compromised.
prcguy

One thing to keep in mind is that SWR does NOT always indicate resonance.

For example, on a perfect quarter wave vertical, the impedance should be 35 ohms resistive, not 50 ohms. This would actually be 1.5:1 swr. Tuning it to 1:1 swr is actually tuning *away* from being the most efficient generally speaking.

I'd also suggest a 1:1 balun / rf-choke at or near the vertical's feedpoint.

 

[LtDoc]

And something else to think about is that all the radial under that antenna affect all the band, not just the ones the radial is tuned/cut for. Unless you have very exceptional 'dirt' to work with, one or two or a dozen radials are just not 'optimal' at all. Certainly better that not having them, but not much.
- 'Doc

 

[nanZor]

The feedpoint impedance will be raised on a vertical when the ground is compromised.
prcguy

Quite true. However in some cases, I've encountered those who actually did improve their ground to a great degree, and then tune away obsessing over a 1:1 when 1.5 was more appropriate with the good ground.

 

[AgentCOPP1]

Okay, so are you guys saying that I should get radials for each band? I could do that but that's even more digging haha.

Hertzian, that'd very interesting. I did not know that 1:1 doesn't necessarily indicate resonance. Damn being on these forums just reminds me of how little I really know!

 

[AK9R]

Do some Googling for "antenna resonance". One of the hits you'll get is an old thread in this forum where zz0468 says (I added some emphasis):

When the inductive reactance value, in ohms, is equal to the capacitive reactance value, in ohms, the antenna is said to be resonant. At that point, the complex impedance is purely resistive. If that resistive value happens to be equal to the characteristic impedance of the feedline, the swr is 1. If not, than the swr is something other than 1. The antenna is still resonant, however.

 

[w9gb]

The Cushcraft AV-4 was their Quarter-Wavelength Vertical (10, 15, 20, 40).
The antenna construction is largely based on the traps for the A3S yagi antenna (and 40 meter dipole).

This model was discontinued in 1999 when Laird purchased Cushcraft.

SECRET SAUCES:

1. Like ALL 1/4-wavelength verticals, you need radials. Manual at Cushcraft/MFJ.
http://www.cushcraftamateur.com/pdffiles/av3482.pdf

2. At the SO-239 connector / feed-point their is a 2:1 UNUN encased on that black insulator.
Excessive power has been known to damaged that part.

w9gb