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Author Topic: Modify a 6BTV ?  (Read 2024 times)
W0NTS
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« on: March 06, 2013, 06:54:20 AM »

Due to space limitations I am unable to put up wire antennas so this limits me to verticals. I presently am using a Zero-5 1040 ground plane which is working very well. I would like to put up a 75M vertical. I have a Hustler 6BTV in storage. Would it be feasible to eliminate the (4) traps used in the 6BTV, keeping only the 75M resonator? The traps would be replaced with aluminum tubing. In theory I wouldn't have to tune it for any band but 75M. I would mount it on the roof of a 24' X 24' garage. Wire radials (4) would be slightly elevated above the pitch of the roof. Perhaps adding a capacity hat could broaden the bandwidth a bit. In effect it seems this configuration would become a 75M ground plane. I would appreciate comments on this idea.  Dennis  W0NTS
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W5CPT
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« Reply #1 on: March 06, 2013, 07:35:46 AM »

What would you gain by removing the traps? The antenna works on 75M as built.  As far as adding a cap hat, I don't think it would benefit you. 

See the following:  http://forums.qrz.com/showthread.php?301344-6BTV-and-adding-a-capacitance-hat

Install it with radials as long and as many that fit and I think it will do fine.

Clint - W5CPT -
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K2DC
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« Reply #2 on: March 06, 2013, 07:58:42 AM »

I agree, you shouldn't have to remove the traps.  In fact, that may only complicate the tuning once you try to mount it.  Also agree that the capacitance hat probably won't help.  Those are to add shunt capacitance to ground (effectively add series inductance) to electrically lengthen a short vertical.  However, because the radials are elevated, you should have at least a couple of them resonant , which won't fit on the roof (you need about 65').  Perhaps you could stretch a few out to nearby trees or other supports.

73,

Don, K2DC
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W9GB
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« Reply #3 on: March 06, 2013, 08:18:38 AM »

Quote from: W0NTS
I have a Hustler 6BTV in storage. Would it be feasible to eliminate the (4) traps used in the 6BTV, keeping only the 75M resonator? The traps would be replaced with aluminum tubing.
A bit of Radio Theory (Physics of Radio), some Facts (How World Works), and Common Sense (Man is not GOD).

The 3.5 to 4.0 MHz radio band (80 meters) is a LONG WAVELENGTH allocation, ~ 80 meters for one wavelength.  SO, antennas have to be Electrically LONG, even if you use various gimmicks / approaches (with compromises) to reduce its physical height or length.

The Hustler 6-BTV vertical Multi-band antenna is a QUARTER-WAVELENGTH vertical radiator (sometimes called a Marconi antenna).  The L-C traps on the BTV serve to SHORTEN the antenna's height, AND permits Multi-band operation. On 80/75 meters, this antenna typically has a usable 50-75 kHz band segment.  Removing L-C traps does not change that restriction.  In fact, you make an antenna that is electrically shorter (de-tuned).

Vertical Antennas, at 80 meters (3.5 MHz- 4.0 MHz), have a NARROW Bandwidth, 50 to 75 kHz segment common.
A QUARTER WAVELENGTH vertical needs to be (80 / 4) = ~ 20 meters (65.6 feet) in electrical height/length.  
That is a TALL Structure !!  Your garage roof can not support a 65 foot radiator for 75 meter phone band.
You then appreciate why AM Broadcasters (550-1700 kHz), at longer wavelengths, use TOWERS as their vertical radiators!
« Last Edit: March 06, 2013, 08:25:22 AM by W9GB » Logged
WB2WIK
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« Reply #4 on: March 06, 2013, 02:46:32 PM »

Due to space limitations I am unable to put up wire antennas so this limits me to verticals. I presently am using a Zero-5 1040 ground plane which is working very well. I would like to put up a 75M vertical. I have a Hustler 6BTV in storage. Would it be feasible to eliminate the (4) traps used in the 6BTV, keeping only the 75M resonator? The traps would be replaced with aluminum tubing. In theory I wouldn't have to tune it for any band but 75M. I would mount it on the roof of a 24' X 24' garage. Wire radials (4) would be slightly elevated above the pitch of the roof. Perhaps adding a capacity hat could broaden the bandwidth a bit. In effect it seems this configuration would become a 75M ground plane. I would appreciate comments on this idea.  Dennis  W0NTS

The radials need to be about 65' long each, they'll extend beyond the roof of a 24 x 24' building.  However, that doesn't mean you can't do that -- it's exactly what I've done with my 6BTV on a roof tower on my own home.  The four 65' long radials don't all fit on the roof, so I just let them run beyond the edges of the roof and tie them off with strings going to trees and bushes and such.

I doubt you'll improve the 6BTV by replacing the traps with tubing.  It's still a very short 80m antenna with a lot of top loading by the resonator.  Mine provides a 2:1 SWR bandwidth of maybe 30 kHz, although I can "push" that to about 100 kHz using a good tuner.  But some use these to operate a particular net which is always on the same frequency, in which case you can tune it for that frequency and be happy. Wink
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W5WSS
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« Reply #5 on: March 07, 2013, 09:00:20 AM »

The 1/4 wave length can be sloped and even push against a tuned pair of radials that are attached and driven from the shielding feed point for best system balance the radials should travel away in opposite directions and can be elevated slightly and ground isolated meaning they are not connected directly anywhere along their entirety and slope downwards towards the anchors.

For best system balance and symmetrical pattern development the antenna should look like an upside down T with the vertical 1/4 wave part centered relative to the wire antenna portion sloped or angled upwards to the high point dead end.

The far ends of the insulated radials can dead end by attachment to an insulated anchor driven into the earth surface.

The top wire tip of the antenna can be fastened via a rope and dog bone insulator to a high point such as a tree limb etc.

A front to back ratio of about 5dbd opposite the angle that looks upwards towards the sky is possible but I found no significant handicap on this band.
The antenna tuned properly and was considered normal bandwidth for the natural (non trap) quarter wave case.

73
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KA1DBE
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« Reply #6 on: March 08, 2013, 06:38:06 PM »

You might try what others have done (And I plan on doing this spring).  Build a 40 meter trap to replace the resonator and then add about 25 or so feet of wire and use it like an inverted L.  I plan on doing that and adding wires for 12 and 17 meters.  If you Google 6BTV Modifications you will find many articles on doing this.  Good luck and 73
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KI4DSC
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Posts: 65




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« Reply #7 on: August 18, 2013, 06:46:25 PM »

Due to space limitations I am unable to put up wire antennas so this limits me to verticals. I presently am using a Zero-5 1040 ground plane which is working very well. I would like to put up a 75M vertical. I have a Hustler 6BTV in storage. Would it be feasible to eliminate the (4) traps used in the 6BTV, keeping only the 75M resonator? The traps would be replaced with aluminum tubing. In theory I wouldn't have to tune it for any band but 75M. I would mount it on the roof of a 24' X 24' garage. Wire radials (4) would be slightly elevated above the pitch of the roof. Perhaps adding a capacity hat could broaden the bandwidth a bit. In effect it seems this configuration would become a 75M ground plane. I would appreciate comments on this idea.  Dennis  W0NTS

So what did you do and how did it work out?

It is tempting to take the traps out as each trap eats up about 1/2 dB.  By time your past all the traps on 75 meter you have lost a lot of your original power to the traps.  But if you take out the traps you will have to add some length to the antenna, how much is determined by the big white load that holds the 75 meter stinger atop the antenna. 

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WX7G
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« Reply #8 on: August 19, 2013, 04:54:54 AM »

The 75 meter resonator is pretty much independent of the length of wire (or tubing) between it and ground. I've used the resonator with tubing from 6 to 33 feet. So go ahead and remove the traps if you like.

Unless you can place several wavelengths of radial wire the radials need be no longer than 1/8 wavelength. See the N6LF radial papers for details. The worse radial length to use is 1/4 wavelength unless you can install more than eight. For the antenna you propose a dozen 20-30 ft. radials should produce a ground loss of roughly 10 ohms. The base-referred loss of the RM-75 resonator is about 20 ohms and so a low loss ground system will not greatly increase radiation efficiency.
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N4UM
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Posts: 453




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« Reply #9 on: August 19, 2013, 08:16:22 AM »

I'd leave all the traps in.  They all provide additional loading for 75M.  if you're unable to extend the radials beyond the garage roof, you might want to consider adding loading coils in the radials.  EZNec will put you in the ballpark so far as inductance values and placement of the loading coils.  The closer you place the coils to the feedpoint, the smaller the required inductance but the less efficient. 
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WB6BYU
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« Reply #10 on: August 19, 2013, 09:01:10 AM »

Quote from: KI4DSC

... each trap eats up about 1/2 dB ...



I think you may find that this might not be an accurate analysis on 75m where the traps are
not resonant.  Instead they will act like small loading coils, and most of the loss will be
in the mobile resonator and in ground losses.


There's nothing magic about starting with the 6BTV:  sticking the mobile resonator on the
end of just about any length of tubing will make a resonant vertical.  And replacing the mobile
resonator with a larger diameter homebrew coil with a big wire top hat instead of a short
whip will improve both the efficiency and the operating bandwidth.
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