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Author Topic: HF6Vx - Using the matching line for a choke or ugly balun?  (Read 3857 times)
KE6TDT
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Posts: 73




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« on: December 12, 2012, 12:01:04 AM »

Those familiar with the HF6V understand the 11'6" matching line of RG11/U comes off the feed point.

This vertical will be roof mounted with it's base about 15' with radials, with about 100 watts out, and no more than 500.  My target is all HF bands but specifically 80 and 40 meters.  From the RG11 matching line I will be using 8x to come into the shack. There is a lot of interior wiring at this venue so I have some concerns.

1. Due to concerns with RF/TVI etc., can I or should I use the matching line to coil into a choke or ugly balun at the feedpoint/base of antenna?

2. If so, should the choke be coiled horizontally or vertically at the base of the vertical? Or does it matter?

3. Would the length of above matching line be long enough to coil into a proper choke?  If not long enough, would it be wise to continue coiling the 8x coax along with the RG11U matching line in order to achieve about 6 turns at about 6" diameter?

4. Would I be better off placing the choke instead of at the feed point, down to  about 60' from the feedpoint, which happens to be just about where it would enter the shack?

5. Last question off topic, if mounted on a roof mounted metal tripod, should I run a short ground wire from the outer conductor of the coax to the tripod? Or would this serve any purpose?

Thanks for the help/advise!

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K3VAT
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Posts: 709




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« Reply #1 on: December 12, 2012, 03:15:38 AM »

Those familiar with the HF6V understand the 11'6" matching line of RG11/U comes off the feed point.
...

The installation of RG-11 is recommended by Bencher only if your completed antenna system shows a high SWR on 20M and all other bands reading within specs.  If one isn't at this point yet, then wouldn't it be premature to assume that this matching line is required?

The best reference and the one that I most often steer folks to with questions about baluns is
http://w8ji.com/verticals_and_baluns.htm

Quote from:  link=topic=86885.msg639542#msg639542 date=1355299264
... My target is all HF bands but specifically 80 and 40 meters.  ...

For halfway decent operation on 80M you'll require at least one pair (preferably two or more pairs) of ~65' radials which should run in opposite directions.  Unless you have a really big roof, one would consider affixing the radial ends to distant trees.

Here's the link to the Bencher reference on when the matching line is to be used:
http://www.bencher.com/pdfs/00366IZV.pdf

Here's the reference link to elevated radials (see right side):
http://www.antennasbyn6lf.com/

GL, 73, Rich, K3VAT
« Last Edit: December 12, 2012, 03:31:37 AM by K3VAT » Logged
WX7G
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Posts: 6052




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« Reply #2 on: December 12, 2012, 06:08:01 AM »

A ferrite choke balun at the feedpoint would be better than an ugly balun because a ferrite balun can provide a high common-mode impedance over a wider frequency range.

The MFJ-915 is available at Ham Radio Outlet for $30. 
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KE6TDT
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Posts: 73




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« Reply #3 on: December 12, 2012, 06:54:02 PM »

A ferrite choke balun at the feedpoint would be better than an ugly balun because a ferrite balun can provide a high common-mode impedance over a wider frequency range.

The MFJ-915 is available at Ham Radio Outlet for $30. 

Thanks for that...But...At the feedpoint?

The manual from the 915 you mentioned, says:   

The RF Isolator should be placed "close to the transmitter", in line with the coax feeding an antenna. This can be done using a coax patch cable .

1. Install the RF Isolator by connecting the coax cable from your
antenna to one end of the Isolator.

2. Connect the other end of the isolator to the transmitter using "a short length of coax".

3. Check to see that the connections are secure.

4. Check the SWR of the antenna using a very small amount of power

What do you think?
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WB6BYU
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Posts: 13249




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« Reply #4 on: December 12, 2012, 07:56:24 PM »

Quote from: KE6TDT

What do you think?




I think it goes to show that some manufacturers don't give good advice about their
products.  Not surprising, given the amount of "lore" floating around that was once
considered state-of-the-art, but really is rather mistaken.

You want the choke as close as possible to the potential source of current on the
feedline because that makes it most effective - that usually means near the
feedpoint.  In the case of the 75 ohm matching section on the HF6V, putting the
balun between the matching section and the antenna may change the match,
depending on the length of coax inside the balun.  In that case I'd put it at the
end of the matching section (though you could wind the matching section around
a suitable ferrite core and make a balun that way.)

Here is an actual example:  at the County Emergency Operations Center we have
an Off-Center Fed Dipole for 40m and 80m, and it was picking up a lot of noise
from the computer network - about S9, making it unusable much of the time.  We
decided to try adding choke baluns to eliminate the common mode current on the
coax, which was causing the coax to act as an antenna as it ran along the network
cables through the building.  We started by putting a choke near the transmitter
in the shack, and it made no difference.  Then we got up on the roof and added
one at a convenient splice and the noise dropped to S4 - S5, a significant
improvement.  We still left the one in the shack as well, just for good measure, but
the one on the roof (as close as we could get to the antenna) certainly made
the most difference.
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N6DMR
Member

Posts: 34




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« Reply #5 on: December 16, 2012, 07:37:16 PM »

If you are wanting to use this antenna on 80 meters, be aware that the bandwidth with this antenna on 80m is very narrow, somewhere between 25kHz - 30kHz.  This is assuming you have a acceptable ground plane or radials.  If you find that the antenna tunes across the entire 80m band, it is an indication of an inadequate ground plane or insufficient radials.

There is a relay switched modification that is on the AD5X website; Phil Silas has a project where 12v relays are used to tap the 80m coil and get 60m as well as a ability to select differend portions of the band at resonance. www.ad5x.com

I have this antenna and I took Phil's article and added two additional relays and can select the entire 80m band by tapping the 80m coil.

One last point, a short vertical like the HF6V is only about 33% efficient on 80m, at 100 watt input you will only radiate about 33 watts.

Duane
N6DMR
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K3VAT
Member

Posts: 709




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« Reply #6 on: December 17, 2012, 10:27:59 AM »

If you are wanting to use this antenna on 80 meters, be aware that the bandwidth with this antenna on 80m is very narrow, somewhere between 25kHz - 30kHz.  This is assuming you have a acceptable ground plane or radials.  If you find that the antenna tunes across the entire 80m band, it is an indication of an inadequate ground plane or insufficient radials.
...
One last point, a short vertical like the HF6V is only about 33% efficient on 80m, at 100 watt input you will only radiate about 33 watts.

Duane
N6DMR

Good points Duane - And this ~33% efficiency for shorten (~1/8 wave) on 80M for antennas such as the HF6V is usually quoted for a ground-mounted antenna system with decent number (~24 radials) of ground mounted radials. (Perhaps someone can assist and actually model this particular arrangement and provide a more accurate efficiency value.)

Since his proposed mounting is roof, then one could infer that the efficiency will be even less with the 'average' setup, i.e., of one or even two pairs of tuned, elevated radials. 

N6LF recommends (see: http://www.antennasbyn6lf.com/design_of_radial_ground_systems/ employing elevated radials in symmetric pairs (each radial ~ 65' long) and only when you get up to 5 or 6 pairs does the elevated vertical antenna system really become optimized (see: http://rudys.typepad.com/files/qex-may-jun-2012.pdf.

So it is quite difficult to design, build, install, and optimize an 80M elevated vertical especially for DX work.  Most of these particular users learn to live with their individual setups knowing that they can often easily talk to the 'locals' and knowing that they're not going to work the ZL9 on the first call, if at all.

GL, 73, Rich, K3VAT
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