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Author Topic: Tuning a Vertical  (Read 4991 times)
WX7G
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Posts: 5908




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« Reply #15 on: March 15, 2011, 02:58:57 AM »

For a 6BTV make them all 40 to 60 ft in length, and put in 20 to 60 of them. 

Jerry, K4SAV

Don't make the radials 40 to 60 feet unless you have lots of wire and time. As I said, read the K6LF radial papers. A couple dozen 20' radials will provide a ground having a base-referred loss resistance of about 15 ohms on 80 meters based on my measurements. It will be lower on the higher bands. Add more 20' radials, 60 total, and the loss resistance will be around 10 ohms on 80 meters dropping to less than 5 ohms on the higher bands. This is a nearly ideal radial system.

If you want to read about the 6BTV vertical and what makes it tick see my article in AntenneX magazine where I measure, model, and analyze it.
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KC0ZPS
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Posts: 48




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« Reply #16 on: March 15, 2011, 05:48:36 AM »

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.

Ok, let me try this.  The feedpoint is 7 inches above the radial field.  I''ll try lowering it tonight when I get home from work.
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NK5G
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Posts: 102




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« Reply #17 on: March 15, 2011, 04:29:48 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

Jerry hit the nail on the head. Think of the radial filed as a "mirror" that the vertical radiating element sees. Current flows in the radial wires which keeps the ground losses down and helps the vertical radiate the RF into the air rather than roasting worms.
In a ground mounted antenna the radials are not the "other half of the antenna", but are a critical part of the antenna system. So the radials will have an impact on the resonnant frequency. Get a good radial field down like Jerry indicated...then adjust the antenna for resonnance.
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W3HKK
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Posts: 593




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« Reply #18 on: March 15, 2011, 05:00:47 PM »

Good luck with the agony of tuning a multiband vert.  We had the same one at field day last year and  with 4 radials for each band and a ground rod, but we couldnt get the swr down on 20m and wound up doing pretty much what you did.  Then FD started and we used it on 40m and 20m....it worked fair to poorly on 40 cw, and worse on 20.  It was a brand new  vert but thi s year we're going to a  80m horizontal loop for  80-10.  The  horizontal 40m loop we put up for the  OSPOTA contest won us first place 2010. 
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KC0ZPS
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Posts: 48




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« Reply #19 on: March 16, 2011, 10:17:44 AM »

W3HKK, you aren't kidding.  So I'm at a point where the optimum SWR is still low on each band.  This means I need to shorten my antenna further, but my antenna is as short is it can currently get.  Anyway, I was thumbing through the eham review of the Hustler 6BTV when I saw this comment by KB7TPB :

"I followed the directions and put down a lot of radials. When trying to tune the antenna I was low on all frequencies and had no more tuning range. Just when I was going to grab my hacksaw and start cutting I found the following site: http://www.hamuniverse.com/hustlerbtvtrapcalibration.html. Do not cut this antenna. Get a good SWR analyzer and follow the directions on the link and it will work well. Good luck."

According to his link, I need to tune the actual traps.  The link he provides even mentions, "When resonance or low SWR dips are far from the correct frequency, use this coarse trap tuning adjustment procedure. Readjusting antenna resonance is easy. When desired tuning cannot be achieved by the adjustments in the Hustler manual, re-tuning the vertical for in-band resonance is best done by adjusting the traps."

"Typical adjustment is about 1/4 to 1/2 inch for a shift of 500 kHz."

...once I again, I'll try and see if this works tonight. Smiley
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WA7NCL
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Posts: 625




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« Reply #20 on: March 16, 2011, 11:18:35 AM »

I would tune for zero reactance on each band starting from the highest frequency.  This is because the traps successively disconnect the upper parts of the antenna the lower you go in frequency.

I would measure directly at the antenna terminals.  Record the data.  You can use the zero reactance frequency to determine if you should make the antenna longer or shorter.  Shorter should raise the frequency.

What i would guess you will find is the zero reactance (resonant) frequency will have a resistance higher than 50 ohms and therefore a high SWR.  The resistance of the antenna over perfect ground should be around 35 ohms.  The amount over this is roughly the ground loss. 

Changing the length of the antenna does not change the high ground loss situation, so getting a low SWR is not necessarily going to happen by changing the antenna length.  Sometimes the feed line length is such that you get some impedance transformation on some bands and might be able to get a low SWR at the transmitter end.  Unless you are a smith chart guru this would be completely by chance.

You  have two options:   Use a tuner and some low loss coax and live with the loss in the feed line due to mismatch (this is the 43 foot vertical approach).

Or add radials until the loss part of the resistance is around 20 ohms giving a total feed resistance of around 50 ohms.

You could also add a matching network to the base of the antenna but I ruled this out because it would have to be switchable for multiband operation.

There is no reason to think you sould be able to magically get a good match on a ground mounted trap vertical with an unknown quality ground system (loss).  Lenghtening and shortening the antenna just changes its feedpoint impedance but there is no assurance you get a 1:1 match for any particular situation.

I think it is interesting to ponder that a 20 ohm loss resistance means that 20/55 or 36% of the power applied is going to warm the worms.  Its also interesting to note that if you make your ground system very good, you will not have a "good" 1:1 SWR in a 50 ohm system because the feed point would be 35 ohms.  So all those vertical owners bragging about their 1:1 swr and broad band operation are just bragging about their ground loss.

My guess is that you will add some radials, get an acceptable SWR under 2:1 and call it good.  If you have a long run of coax or its lossy, just invest in something a little better.  Then get on the air and have fun....
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KC0ZPS
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Posts: 48




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« Reply #21 on: March 16, 2011, 05:20:56 PM »

Ok, finally got it!  Talk about a learning experience...

After following the directions on this page :

http://www.hamuniverse.com/hustlerbtvtrapcalibration.html

I managed to get my SWR readings below 2.0 an almost all of the bands.  The 40m band is still a bit high, but I'm just going to deal with it.

FrequencySWR
3.80012.7
3.9421.4
4.0005
7.1751.4
7.3002.2
10.1001.1
10.1501.3
14.2251.3
14.3501.2
21.2751.3
21.4501.1
28.3001.3
29.7001.5
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KB8UUZ
Member

Posts: 37




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« Reply #22 on: March 17, 2011, 09:52:02 AM »

Hustler BTV's (4,5,6) - As previously posted by WX7G, KC0ZPS and NK5G - Go to the DX Engineering website and get the 60 page manual (download it for FREE):

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

This manual has ALL the information you need on tuning (Chapter 7) the BTV antennas!  Trap calibation is IMPORTANT!

Radials - basically get as many as you can in the space you have available - have them radiate from the antenna like the straight spokes on a wheel. Don't have them cross over each other & don't use chicken wire.....

Follow thw direction in the DX Engineering maual and the Hustler BTV will really perform well!

Tom, KB8UUZ

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N8CMQ
Member

Posts: 353




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« Reply #23 on: March 30, 2011, 05:04:19 PM »

Here is what I did for a radial field, got to go to the first blog entry.

www.myspace.com/n8cmq

Not saying you have to copy it, but the idea is to create a mirror surface on the ground and eliminate losses.
And while some say short radials are OK, the longer the better, especially for low angle radiation.

This is my second radial field, and it is over twice as long and three times the number of wires!
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G3TXQ
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Posts: 1432




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« Reply #24 on: April 01, 2011, 04:11:07 AM »

And while some say short radials are OK, the longer the better, especially for low angle radiation.

If you consider the geometry you'll see that the Fresnel Zone where low elevation angle ground reflections occur extends a considerable distance from the base of the vertical - much further than we could typically install radials. To quote from the ARRL Antenna Book:

Quote
In short, far-field losses for vertically polarized antennas are highly dependent on the conductivity and dielectric constant of the earth around the antenna, extending far beyond the ends of any radials used to complete the ground return for the near field. Putting more radials out around the antenna may well decrease ground-return losses in the reactive near field for a vertical monopole, but will not increase radiation at low elevation launch angles in the far field, unless the radials can extend perhaps 100 wavelengths in all directions! Aside from moving to the fabled “salt water swamp on a high hill,” there is very little that someone can do to change the character of the ground that affects the far-field pattern of a real vertical.

73,
Steve G3TXQ
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N8CMQ
Member

Posts: 353




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« Reply #25 on: April 01, 2011, 05:10:52 AM »

Then don't install any radials on your antenna, doesn't matter...

 I will still add as much wire I can on my system, I can see the difference here.
Perhaps theory and practice, or reality and NEC can have differences?
The poor soil I have here in Michigan needs all the improvement I can give it, as the glacial deposited sand is very ohmic and turns RF into heat.

How I raised the antenna radiation resistance to help might not interest anyone though...
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G3TXQ
Member

Posts: 1432




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« Reply #26 on: April 01, 2011, 05:28:35 AM »

Quote
Then don't install any radials on your antenna, doesn't matter...

I don't think anyone is saying that radials don't matter - clearly they do. They improve the soil conductivity in the reactive near field, which in turn reduces losses and improves efficiency.

The point I made - supported by a simple look at the geometry, and by N6LF's section in the Antenna Handbook - is that the radials would need to extend outwards many tens - perhaps hundreds - of wavelengths in order to alter the elevation pattern at low launch angles.

If you install extra radials and notice an improvement, unless those radials are many wavelengths long the improvement is largely coming from increased efficiency not from a change in shape of the elevation pattern.

73,
Steve G3TXQ
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N8CMQ
Member

Posts: 353




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« Reply #27 on: April 01, 2011, 02:20:51 PM »

Sorry, kinda sensitive on the subject. I have had many disparaging remarks by people that have not bothered to try to build a radial field and measure the differences... If they would look at my blog on myspace, they would see I have a lot of time and wire involved in mine.
While it is difficult to measure very low angle radiation, I still believe 1/8 and 1/4 wave surface mounted radials improve vertical polarization low angle radiation. It may not be perfect, but I find a vast improvement over a horizontal half wave over bare soil.
At this stage of the game, we are repeating the work of others and refining it, and to hear the same old wives tails repeated as gospel gets a little frustrating...
Normally I have a thicker skin...
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