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Author Topic: Hustler mobile antenna  (Read 2082 times)
G0VQY
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« on: May 11, 2004, 02:47:27 PM »

Hello everybody,

I have just purchased a 20 metre hustler mobile antenna.  I have it mounted on a bracket that is welded to the chassis at the rear of the car just below bumper level.  I can only get the SWR down to 1.8.  I wonder if this is normal with this kind of antenna or is there a way that I can get it down a little bit lower.  I have not done a lot of mobile operation so I could do with as much advice as possible.

Thanks a lot

G0VQY
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WB2WIK
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« Reply #1 on: May 11, 2004, 03:01:52 PM »

Which mast do you have?  Is it an RM-20 or RM-20S model?

Is 1.8:1 the lowest it will achieve after continuously varying the whip length above the resonator coil?  Can you tell if it's reactive, and if so is it capacitive or inductive?  

WB2WIK/6
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AA4PB
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« Reply #2 on: May 11, 2004, 03:41:17 PM »

My experience with the Hustler is that 1.8 to 1 is about right. The impedance in most good installations is around 25 ohms (2:1 SWR) so you need some sort of matching at the base. I used a 2:1 UNUN with good success. The UNUN has the benefit that you don't need to change it when you change resonators for a different band.
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G0VQY
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« Reply #3 on: May 12, 2004, 07:20:24 AM »

Thanks for the reply.  I have got the and MO-3 mast and am using it in conjunction with the 20 metre super resonator.  Yes, we can only get the SWR down to 1.8.  I cannot answer your question about whether the antenna is reactive.  Anyway, any advice on how I can improve the station would be appreciated.
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K0BG
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« Reply #4 on: May 12, 2004, 09:47:33 AM »

If you want more information on how to build an unun for HF mobile, visit my web site at http://www.k0bg.com and click on Ununs.

Alan, KØBG
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W7DJM
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« Reply #5 on: May 13, 2004, 11:28:13 AM »

the fact is, the antenna ITSELF may not "need" to be improved, but we know what you meant.

There are too many variables, here to know just what is going on, but here goes.

First, at resonance, NOTHING ANYWHERE says that any of these mobile antennas present 50 ohm impedance, or 1 swr.   This antenna may be at resonance, non reactive, as good as it gets, but in view of how, where it is maounted, that just may be what you have.  Mobile whips, generally, are far less than 50 ohms.  

What you need to do, assuming the coax is grounded well and so on, is employ some sort of base matching device.  You can either add some capacitance to ground, or add a small inductor to ground, and tap your feed point up from ground on the coil.
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AA4PB
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« Reply #6 on: May 13, 2004, 01:58:33 PM »

The radiation resistance of an 8-foot whip on 20M is only about 7 ohms. The load impedance is the radiation resistance plus the loss (coil, wire, ground). If you have an 8-foot whip with a 1:1 SWR (50 ohms) without some type of base impedance matching, that means you have 43 ohms of loss resistance. 86% of your RF power is being turned to heat and only 14% of your power is being radiated as signal.

Given no impedance matching at the base, a more efficient antenna will have a higher SWR. My experience with 20M Hustlers is that when properly installed with good grounding they run somewhere around 25 ohms (7 ohms radiation resistance and 18 ohms of loss). If someone tells you they get a nice 1:1 SWR with no impedance matching that means they have an extra 25 ohms of loss somewhere and their Hustler is only half as efficient as one whose impedance is 25 ohms. If we could eliminate the loss entirely then the antenna would be 100% efficient (it would radiate all the power) but the impedance would be 7 ohms and it would have about a 7:1 SWR without any impedance matching.

So, there are two issues with a mobile HF whip. We must make it resonant on the operating frequency and then we must provide impedance matching to get the SWR down to something reasonable in order to minimize loss in the coax and make sure the transmitter can load it okay.

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G0VQY
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« Reply #7 on: May 13, 2004, 02:23:39 PM »

Thanks for all your help guys. I've taken it all on board.

http://www.wheelchairanglingandhamradio.co.uk/radio.htm
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K8KAS
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« Reply #8 on: May 21, 2004, 01:09:41 PM »

All you need is about 100 to 150 Pf mica cap to ground from the base of the antenna (20 meters). This is a common matching method refered to in the handbooks. I use this on almost all of my Hamsticks and Hustler antennas. I believe MFJ sells a box with switched caps for this purpose as well.
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M0DSF
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« Reply #9 on: May 29, 2004, 01:01:14 PM »

I can't help you with your problem but perhaps you can just tell me something. I'm thinking of buying an RM20(S) but I can't find out how long they are! All the catalogues say is the RM10-RM80 are various lengths up to 1m. I need to able to pack it into a bag so if it's more than about 70cm it won't do - though I suppose I could cut it and put a joint in it. Does the top whip detach from the resonator?

Thanks

Doug.
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G0VQY
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« Reply #10 on: June 02, 2004, 08:08:15 AM »

Hi

Yes you can unscrew the whip from the resonator.

73

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