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Author Topic: Tuning a Vertical  (Read 9917 times)
KC0ZPS
Member

Posts: 52




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« on: March 13, 2011, 07:58:01 AM »

How do I tune a Hustler 6BVT antenna or do I other issues? Here are the readings for my antenna. 

FrequencySWRResistanceImpedence
3.8006.566140
4.00013.5135325
7.1753.72144
7.3005.52270
10.1005.8910
10.1506.1913
14.2253.41515
14.3503.91522
21.2752.61912
21.4502.2155
28.3001.55224
29.7002.22410

These readings are at the base of the vertical.  I bypassed my 100' coax because I was getting around the same readings at my radio and I wanted to eliminate the coax cable.  I am using radials, 2 for each band.

Oh, ignore the 80 meter band.  I did not adjust that correctly just yet, but that shouldn't affect the other bands.

So back to my original question.  Do I need to tune the antenna (and if so, how) or do I have other problems with the antenna itself.  Basically, I understand that the vertical needs to be lengthened or shortened.  However, for those with the Hustler antenna, which part of the antenna below the trap do I adjust?

For example, on the 10 meter band, I can lengthen the antenna just below the 10 meter trap, or just above the antenna base.

For the 15 meter band, I can lengthen the antenna below the 15 meter trap, or above the 10 meter trap.

For the 20 meter band, I can lengthen the antenna below the 20 meter trap, or above the 15 meter trap.

Which do I lengthen and by how much??

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WB6BYU
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Posts: 17279




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« Reply #1 on: March 13, 2011, 09:20:09 AM »

First ignore the "resistance" and "impedance" columns.  Unless you can read out the
sign of the reactance (plus or minus), your best bet is simply to tune it for minimum
SWR in the desired part of each band.  (Knowing if the reactance is + or - may be
useful when you are measuring RIGHT AT the base of the antenna, but doesn't help
at all with any length of coax unless you apply corrections for the coax length.)

You can adjust whatever joint in the antenna is easiest, but always below the trap
for the band you are tuning.  I don't have one handy to look at, but I think the
one closest to the trap may be a good choice - the trap should be attached to
the thinner tube that slips inside the outer one.

Depending on where the dip is on 10m, it looks pretty close.  Always start at 10m
and work down in frequency, as the settings of the lower sections affect the
tuning of the higher ones.  Presuming that the tuning of 10m is acceptable (which
would depend on the SWR at 28.5 to 29 MHz probably) it looks like 15m is too
short and 20m is too long.  You'll have to see what 30m and 40m look like after
making those adjustments.

By how much?  The manual might give you some idea of how many kHz per inch
of adjustment on each band, but you'll just have to try it and see.

Make sure that your radial system is installed and properly connected to the antenna
before tuning - that affects the settings, and otherwise you'll have to retune the
antenna once the radials are installed.
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KC0ZPS
Member

Posts: 52




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« Reply #2 on: March 13, 2011, 01:56:05 PM »

Thanks for the reply WB6BYU.  I've spent the last hour attempting to tune the antenna.  In the end, I ended up shortening the vertical by over a foot.  In fact, the antenna can't physically get any shorter.  These are the readings now.  I assume this is acceptable.

FrequencySWR
3.80010.3
4.0002.1
7.1751.7
7.3003.3
10.1004.2
10.1504.7
14.2252.9
14.3503.3
21.2752.6
21.4503.0
28.3001.8
29.7002.3

The 80 meter band is tuned towards the upper end of the spectrum.
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KH6AQ
Member

Posts: 7718




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« Reply #3 on: March 13, 2011, 05:44:29 PM »

See DX Engineering for a paper on tuning the BTV verticals.
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KC0ZPS
Member

Posts: 52




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« Reply #4 on: March 14, 2011, 08:25:54 AM »

See DX Engineering for a paper on tuning the BTV verticals.

Thanks.  I found the article and was reading over it.

http://www.dxengineering.com/pdf/BTV-INSTALL-GUIDE-Rev0a.pdf

The section about the radials is interesting.

"On ground-mounted quarter-wave verticals, the length of the radial wires is not critical. They can be as short as one-eighth wavelength, but one-quarter wavelength radials on the lowest band of operation are typical."

"You may use wires of any convenient length. Just run your radials in as many directions as you can, straight away from the antenna base taking care not to zig-zag, bunch or cross over other radial wires"

I guess I'll be installing more radials this weekend.  Right now, I'm only doing two radials which are cut for each band.  From the article, it sounds as if the length doesn't matter for ground mounts and the more you have, the better.
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WB6BYU
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Posts: 17279




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« Reply #5 on: March 14, 2011, 08:38:09 AM »

Quote from: KC0ZPS
Right now, I'm only doing two radials which are cut for each band.  From the article, it sounds as if the length doesn't matter for ground mounts and the more you have, the better.


That is correct.  For an elevated antenna, fewer radials will work but they have to be resonant,
and the radial length affects the antenna tuning.

For a ground mounted antenna (which should have the feedpoint within a few inches of the radial
system) the exact length isn't as important, but you need more radials to maintain efficiency.
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KH6AQ
Member

Posts: 7718




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« Reply #6 on: March 14, 2011, 11:21:34 AM »

If the antenna is ground mounted the worst radial length to use is 1/4 wavelength. Ground losses can actually be increased unless eight or more radials are used. The K6LF papers detail this.

Unless you are installing more than eight wavelengths of wire (1100' at 7 MHz) you get more bang for your buck using 1/8 wavelength, rather than 1/4 wavelength, radials. With 1200' of radial wire (12' to 20') I measure 5-10 ohms of ground loss depending on the band.

If you're installing radials on a lawn the DX Engineering biodegradable plastic lawn staples are a good way to go. They are large and are designed to be pushed into the ground using the palm of your hand. I installed 300 yesterday for 1000' of additional radials and it took less than one hour to apply the staples. One staple placed every 3-4 feet works well. Cut the lawn very short, lay the radials, and staple. Then set the lawn mower height to high for a few months. The radials will be completely covered and difficult to locate. In a year or two the staples will be desolved. The staples are $16 for 100.
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W5DC
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Posts: 146


WWW

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« Reply #7 on: March 14, 2011, 02:41:04 PM »

When the radials are placed very close to, on, or beneath the ground, the ground acts as a lossy dielectric material.  The dielectric properties of the ground effectively lowers the resonant frequency of the radials, thus requiring them to need shortening for resonance.  Because the ground is lossy, the resonances of the radials become very/extremely low Q making tuning them for resonance unimportant and impractical in most cases.  Furthermore, in the same situation, the currents induced in the ground can substantially increase the antenna losses.  You can use either a small number of elevated radials at the cost of increased complexity and clutter, a small number of radials close to, on, or beneath the ground radials at the cost of increased losses, or a much larger number of radials close to, on, or near the ground.

In many cases such as mine, the only practical radials are one or two elevated radials per band. Use what's appropriate for your circumstances.

Read all the articles by Sevick.

Dunc, W5DC
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NK5G
Member

Posts: 105




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« Reply #8 on: March 14, 2011, 04:01:13 PM »

A properly tune btv with a radial field should have an SWR of about 1.3:1 as the antenna is not 50 ohms.

As indicated in other posts, use the instructions from DX engineering and you won't go wrong.
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KC0ZPS
Member

Posts: 52




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« Reply #9 on: March 14, 2011, 07:24:22 PM »

Ok, so I messed around with the analyzer a bit more today and found something interesting.  The SWR measurement looks like a bell curve.  The lowest point I found for each band is :

BandLowest SWR PointSWR
40M7.1131.3
30M9.7601.1
20M13.7101.1
15M20.5751.0
10M27.0001.0

In each case, the best case SWR is below the section of band I'd like it to be so when I increase the frequency, my SWR increases.  With this knowledge, does it mean my antenna is still too long?
« Last Edit: March 14, 2011, 07:28:29 PM by KC0ZPS » Logged
WB6BYU
Member

Posts: 17279




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« Reply #10 on: March 14, 2011, 07:29:42 PM »

Yes - to raise the resonant frequency you need to shorten the antenna.
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K4SAV
Member

Posts: 2411




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« Reply #11 on: March 14, 2011, 08:58:43 PM »

The length of the radials on the ground is not critical ONLY if you have an adequate number of radials.  Two radials is far from adequate.  When there are only a few radials on the ground, they will appear mostly resonant and a little lossy.  The ground lowers the resonant frequency of the radials a lot, so if you cut them to 1/4 wavelength based on some formula they will actually be resonant at a much lower frequency.  That will lower the resonant frequency of the antenna, which is the effect you are measuring. 

Trying to cut radials for a certain frequency when they are on, or in, the ground is a waste of time and probably won't be correct anyway.  You would actually have to measure the resonant frequency of the radials, but this is also a waste of time.  Gain will be very poor with only two radials on the ground.  For a 6BTV make them all 40 to 60 ft in length, and put in 20 to 60 of them.  After you do that your antenna should start acting normal again.  You will have to go back and undo the tuning you did to make the antenna shorter.  Performance should much better also.

Jerry, K4SAV
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KC0ZPS
Member

Posts: 52




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« Reply #12 on: March 14, 2011, 09:13:49 PM »

K4SAV, just to clarify, I have two radials per band (12 total).  I had a typo in my initial description...

WB6BYU, I was hoping you wouldn't say that.  My antenna cannot get any shorter, so I'm not sure what to do at this point to raise the resonant frequency.
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K4SAV
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Posts: 2411




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« Reply #13 on: March 14, 2011, 09:44:06 PM »

Two radials per band means there are effectively only two radials on the lower bands.  That is why your antenna is not resonant on the right frequency, as I tried to explain above.  You can eventually get it to the right resonant frequency by shortening the vertical element (although you might have to cut some aluminum) but the performance will be very poor.  Putting in a good radial system is the right way to solve the problem.

Jerry, K4SAV
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WB6BYU
Member

Posts: 17279




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« Reply #14 on: March 14, 2011, 09:51:08 PM »

Try it again after you put in the new radial system.

How far is the feedpoint of the antenna above the radial field?  That counts as part of the
antenna length, and if it is too far the antenna won't tune.

I think others have pointed you to the instructions for retuning the traps if you can't shorten
the antenna far enough.
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