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Author Topic: Hustler Mobile Mast Length  (Read 1810 times)
KL7YK
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« on: May 25, 2010, 08:21:02 PM »

Looking for anyone who has already tried this.

I have been using Hustler HF mobile antenna for years, have no issues with them.  I run the 54 solid mast, have resonators for 75, 40, 20 and 15m.  Even have a duplicate setup on the boat.

What I am looking at is does lengthening the mast add anything to the effectiveness of the antenna?
Yes, I know the $800 screwdriver is the cats meow....but I have what I have.

I am playing with a 24 inch addition to the basic 54 inch mast.  So I have a 78" mast section below the resonator.  Tunes fine, radio is quite happy.  Have not put an analyzer on this yet (don't happen to own one) so I could be simply creating a better dummy load.  I have other lengths of extensions to play with, longer still.  Poor bands of late for Alaska have limited my on air checks with anything "DX".
But if you have tried this or can model this let me know.

Ron, KL7YK
Anchorage
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K0BG
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« Reply #1 on: May 26, 2010, 07:27:10 AM »

Where you want to add the length is at the top (above the coil), rather than to the bottom. While length matters wherever you add it, putting it on the top raises the current node. Or, you could use a properly placed cap hat. However...

The Hustler coils are not adjustable, and therein lies the biggest problem. With no way to adjust the coil's reactance, you have to adjust the whip length, and that's not always easy. You can use say a 15 meter coil on 20 meters by extending the whip length, but the trick doesn't work for a 20 meter coil on 40 meters.

By the way, stick with the smaller coils as their Q is better than the big ones due mainly to the large end caps on the larger coils.

After you play with the Hustler for awhile, you'll see why so many folks opt for a screwdriver.
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WX7G
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« Reply #2 on: May 27, 2010, 10:53:09 AM »

Yes adding length below the resonator is the place to do it. You are increasing the "current-area" of the antenna, which is current integrated along the antenna length.

For example, a Hustler mobile antenna mounted on a perfect, flat ground plane and having a 30" whip above the coil and a 54" mast below the coil has a current area (with 1 amp at the base) of 69 amp-inches. For simplicity I have not included the resonator length.

Now let's add your 24" mast so we have 78" below the coil and 30" above. The current-area is now 93 amp-inches. The radiation resistance has increased by the square of 93/69 or 1.8 times. At 7 MHz the radiation resistance has gone from about 4 ohms to 8 ohms.

The GND and coil loss resistances now consume less of the RF power and more is radiated. Given zero GND and zero coil loss nothing is gained. Given very lossy GND the signal will increase by 3 dB. In a real world installation with finite GND and coil loss and the vehicle current-area figured in the increase in signal at 7 MHz might be 2 dB.

The current distribution below the resonator is constant. That is why a 78" mast with 1 amp has a current-area of 78 amp-inches. The current distribution above the resonator is triangular. That is, it is 1 amp at the base and 0 amps at the end. The current-area is 1/2 x length x current. For the 30" whip the current-area is 1/2 x 30" x 1 amp = 15 amp-inches.

Mounted on a car the vertical length of the vehicle below the antenna base adds to the current-area.  That is one of the reasons why getting the antenna high on the vehicle increases performance. The car radiates RF and is part of the antenna.

The far-field electric field contribution of each part of an antenna is proportional to the current-area. The 78 amp-inch portion produces 5X the electric field that the 15 amp-inch portion does.
« Last Edit: May 27, 2010, 11:01:50 AM by DAVE CUTHBERT » Logged
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