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W9JI
02-21-2004, 11:56 PM
I have been reading information on buying a screwdriver antenna. I came across this article on the net and the guy came up with a way to tune a screwdriver better but does not know why it works. Maybe someone can explain it here so I understand.

Add a matching stub in-line with your screwdriver #antenna. IT WORKS! Here's how to make one. You need to start with a 6 foot length of coax with these specific properties:

75 Ohms
Velocity factor of 0.66
I used Belden 8241 RG-59/U Type E108998 60C IC 23 Shielded cable.
Steps:

Cut the coax to a length of 68 inches.
Connect a PL259 connector to each end of the stub ( or use whatever connectors you currently have connected to your antenna). NOTE: Overall length of stub shall remain at 68 inches tip-to-tip.
Get a barrel connector to connect the new stub to the old coax (feed line).
Connect the 68-inch stub to the antenna feed point.
Connect the coax from the radio to the opposite end of the stub using the barrel connector.
That's it! You're done! Now check the tuning. You will be pleasantly surprised.
Before I made the stub, I was a bit skeptical about what a little piece of coax would do to improve my set up but here are my numbers before and after the installation of the stub.

Before:

I could tune as low as 3.775 MHz without coil running off the screwdriver threaded rod.
Tuning on all bands was between 1.5:1 to 2:1 SWR - 17 and 20 meters tuned the best and 10 meters tuned the worst.
Tuning point for 10 meters was between the 40 meter and 75 meter tuning points - where the antenna was a wave length for the 10 meter frequencies.
After:
I can now tune as low as 3.550 MHz with a slight margin to spare.
Tuning on all bands is now between 1:1 and 1.5:1 SWR, at 100w power.
Tuning point for 10 meters is now below the 12 meter tuning point, in-line to where you would expect it to be, at the low end of the screwdriver run.
So, for a minimal outlay of cash, about $6, you can get a marked improvement in the operation of your screwdriver antenna without taking it apart.

I am not an electrical engineer and will not begin to tell you why this works, but it does. If anyone knows why this matching stub works so well, please tell me and I will include the explanation here.

Authors Note:
My antenna is mounted to the frame supporting the rear bumper of a 1996 Ford Explorer. The main coil of the antenna is therefore lower than the roof of the vehicle. This, I am sure, affects my SWR readings. Your mounting situation may be different from mine, so I cannot guarantee the results you will receive. The effects of using this stub may vary considerably due to your own particular circumstances.

W8JI
02-22-2004, 01:26 AM
</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (Happy_Hamer @ Feb. 21 2004,16:56)</td></tr><tr><td id="QUOTE">I have been reading information on buying a screwdriver antenna. I came across this article on the net and the guy came up with a way to tune a screwdriver better but does not know why it works. Maybe someone can explain it here so I understand.[/QUOTE]<span id='postcolor'>
Sure&#33;

It can be explained by pure luck.

He disturbed the system by doing something that makes no sense at all and should not have a major effect at all, and it made his system appear to work the way he likes it.

Could be a thousand different reasons, like maybe he had a bad coax connection and he fixed it or maybe a bad ground and he fixed it. Whatever it is, there sure isn&#39;t any reason for planning on it to work in other cases.

73 Tom

AE4FA
02-22-2004, 03:26 AM
Well, IF the antenna were resonant at 41.29MHz, and IF the characteristic impedance were 100 Ohms, it would make sense. The section of 75 Ohm coax would act as a matching section to a 50 Ohm feedline.

But that&#39;s not what&#39;s presented. Tom&#39;s right. That&#39;s gibberish.

WA2ZDY
02-22-2004, 03:37 AM
The luck of the draw, or a happy accident - if indeed it&#39;s an improvement.

Keep in mind, if you run a long enough feedline to any antenna, or even to NO antenna at all, you&#39;ll see a flat SWR. The reflected power gets lost in the feedline loss and it doesn&#39;t show up back at the SWR bridge. But it certainly doesn&#39;t work very well.

So thus I say &quot;IF indeed it&#39;s an improvement.&quot; It may not really be one.

AE4FA
02-22-2004, 03:53 AM
</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote </td></tr><tr><td id="QUOTE">I say &quot;IF indeed it&#39;s an improvement.&quot; It may not really be one[/QUOTE]<span id='postcolor'>

Good point, Chris&#33;

Low SWR is not the be-all and end-all for antenna systems.

W8JI
02-22-2004, 08:35 AM
</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (ae4fa @ Feb. 21 2004,20:53)</td></tr><tr><td id="QUOTE"></span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote </td></tr><tr><td id="QUOTE">I say &quot;IF indeed it&#39;s an improvement.&quot; #It may not really be one[/QUOTE]<span id='postcolor'>

Good point, Chris&#33;

Low SWR is not the be-all and end-all for antenna systems.[/QUOTE]<span id='postcolor'>
The odd thing about this &quot;cure&quot; is a mobile antenna, when working properly, has an impedance LOWER than 50 ohms.

Adding ANY LENGTH of 75 ohm cable in series with the antenna can only INCREASE the antenna impedance, it can never reduce it. (It can only reduce the antenna impedance of the cable is LESS than the antenna impedance. For example a 100 ohm quad element can be matched to 50 ohms with 1/4 wl of 75 ohm cable.)

That means adding the suggested 75-ohm section could only make the antenna SWR increase if the antenna was behaving normally, and had an impedance 50 ohms or lower. Anyone with a mobile antenna impedance over 50 ohms has a major problem in the antenna, feedline, or grounding.

73 Tom

KM5FL
02-22-2004, 09:14 AM
If SWR of 1:1 is really the &quot;be all and end all&quot; of an antenna, I have the worlds best.. It&#39;s called a Cantenna and it&#39;s made by Heathkit...

KM5FL

AG3Y
02-22-2004, 08:06 PM
Sounds like the old CB &quot;Prunin&#39; the feedline for best VIZWAARS&quot; mumbo-jumbo to me &#33; Will the old urban legend ever die ?

73 from Jim

NZ3M
02-23-2004, 01:44 AM
</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (ae4fa @ Feb. 20 2004,21:53)</td></tr><tr><td id="QUOTE">Low SWR is not the be-all and end-all for antenna systems.[/QUOTE]<span id='postcolor'>
You are exactly right, my dummy load has a perfect SWR. http://www.qrz.com/iB_html/non-cgi/emoticons/wink.gif

Dave

K7FE
02-23-2004, 02:25 AM
Tom, W8JI
Hi-Q and some other hf mobil antenna manufacture&#39;s use an adjustable tap method to tune mobil antenna coils which creates a &quot;shorted turn&quot; effect. Have any antenna modeling guru&#39;s posted on the web data on efficiency&#39;s between screwdriver type tapped coils and coils cut to the exact length like the old Master Mobil monobanders?

Other screwdriver antenna manufactures move a conductive tube over the unused coil turns to reduce the mutual coupling (&quot;shorted turn&quot;) effect and improve efficiency. How well do you think that works considering that the end of the tube next to the coil is open?

73,
Terry, K7FE

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