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Author Topic: screwdriver impedence matching  (Read 1843 times)
KC8NTP
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Posts: 16




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« on: February 14, 2007, 05:49:25 PM »

I am going to putt a Yaesu FT857 in my truck ('94 Chevy extended cab) and I am pondering which antenna to use. The antenna that I have come up with is an MFJ 80m-6m screwdriver.
I am going to take the whip that it comes with off and use a 102" stainless steel whip. I am also going to use some solid 10 gauge wire to make a cap hat.

This shows real promise (improvements of 3db on 20m, 4db on 40m, and 6.5db on 75m over stock), other than the fact that below 20m I need some way to match the impedence. On 20m the feed point impendence is around 40 ohms using this setup, which is fine, but on 40m it is 22 ohms and around 15 ohms on 75m.

Oh, and these numbers are from simulations that I have ran using EZNEC and NEC2 - and there is not much difference between the programs on their predictions.

For matching the antenna I have thought about getting one of those little black boxes from MFJ that have multi impedences for antennas down to 2 ohms, but I want to know if anybody has used one with any kind of success before I try one.

I know that this won't be the best setup out there, but I know that I should be able to get out fairly well on 40m-12m, and make some contacts on 75m - I know that with the cap hat and the longer whip that it will be a little long for 10m, so I am going to use a seperate whip for 10m.

If somebody out there could tell me if this is a good setup, or if I could do something different, I would greatly appreciate it.

I have ran 10m, 12m and 15m mobile before; using a 102" whip with an adjustable cap hat, but I have never tried any of the other bands before.

John KC8NTP, east central Nevada
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AI4NS
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« Reply #1 on: February 14, 2007, 07:00:45 PM »

Go to Alan's website: www.k0bg.com. Must read for HF mobile. Don't believe everything you read. It is hype. Mobile antennas have no gain, only more or even more loss. Bonding everything is important to have a successful HF experience.

Mike
AI4NS
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K0BG
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« Reply #2 on: February 14, 2007, 07:06:33 PM »

Well, it would be nice if it was a simple as you project, It isn't!

Inductive matching has a lot of merit for a lot of reasons, but it is not the only way to skin the cat. An average mobile antenna (depending on length and frequency of operation), exhibits a capacitive reactance of between 15 pF and 45 pF. This reactance must be cancelled with an appropriate inductive reactance in the form of a loading coil. Variables include the over all length, coil Q, resistive components, and most of all, ground losses. All else being equal, the best way is to use inductive loading.

Rather than go into detail here, I suggest you visit my web site and read a few of the articles dealing with antennas and matching.

Alan, KØBG
www.k0bg.com
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N3OX
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« Reply #3 on: February 14, 2007, 07:17:23 PM »

"Well, it would be nice if it was a simple as you project, It isn't! "

Replacing the stock whip on a screwdriver with a longer, hatted one isn't as simple as replacing the stock whip on a screwdriver with a longer, hatted one?

- - - - - -

KC8NTP: how does the original MFJ screwdriver deal with this impedance matching issue?  Does it already have a UNUN box?  Does it use some of the coil for impedance step-up?

The resistive part of the impedance is going to be smaller with the stock whip than with the longer one.

I wonder though, did you include the resistivity of the SS whip in your EZNEC model?

The relative increase in gain won't be so much if you didn't.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KC8NTP
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Posts: 16




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« Reply #4 on: February 15, 2007, 02:23:37 AM »

Everybody's input has been helpful. No, I did not even think about the resistance of the whip; However, I did go back and change some things after I read everone's replys, which it did not change things much.
Oh, and one person mentioned that mobile antennas have no gain. The numbers that I was stating was gain over the stock antenna; it still would be about -7dbi @ 20 degrees on 75m.

I am a major antenna nut, however, I have never played around much mobile except for 70cm-15m. Now when it comes to matching a mono bander, or even a homebrew triband trap vertical I am there. However, I know that matching a screwdriver or any complex multi band mobile antenna takes a little more knowledge than I have. I know that I may have to use an impedence match box plus an adjustable shunt, and I have heard that this is a pain that happens to be mobile HF.

And, Alan, the MFJ whip uses 3/8" X 24 threads on the whip, so that part is straight forward - I think that is what you where meaning by replacing the whip.

I do need to find my way back to Alan's website - I have been there a few times, but it has been a while; actually I have e-mailed Alan a few times on high power mobile setups, which he is "the man" when it comes to running mobile, so I take his advice to heart.

John KC8NTP, east central Nevada
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KC8NTP
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« Reply #5 on: February 15, 2007, 02:30:25 AM »

That was Dan that said something about changing the whip. Don't mind me: 19 hour day, with screaming 3 year old, so brain not 2 gud
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N3OX
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« Reply #6 on: February 15, 2007, 03:48:41 PM »

John,

What does the MFJ antenna use right now?

Does it have a shunt coil like the one on Alan's site?
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KC8NTP
Member

Posts: 16




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« Reply #7 on: February 15, 2007, 11:52:51 PM »

Dan:

The MFJ screwdriver is as basic as they come. They actually use a telescoping whip and do not come with any kind of shunt coil. The base, as well as the whip, uses 3/8" X 24 threads so changing the whip is easy.
At the base of the antenna there is a wing nut so you can hook up a capacitor or inductor, which makes it easy to match it, I guess. The thing that I am trying to accomplish, is to be able to match the antenna from band to band with easy. I was going to use the Yaesu ATAS 120, but after I heard about all the problems that they have I said screw that, pun intended.

I guess that I can setup some matching networks for the low bands; having the wing nut that connects to the coil would make this easy - since I would have to get out of the truck to change the coil setting anyway; just keep inductor/capacitors in my tool box and have them marked for the band.

This all seems like a pain in the butt, but I think that I can find a way to make it work. I also know that getting it to work right is going to involve getting everything bonded correctly. I have ran high power mobile (1500w on 10m, 12m, and 15m) from a pickup truck, so I already know some of the grounding issues that can come up. The hardest time that I have had with grounding was actually a Geo Metro hatchback. However, my Ford truck used to look like a maze of ground wires - mostly from the box to the cab, and everything was tied to the frame. I still managed to toast more than a few voltage regulators.

I am not looking at using high power with this setup that I am about to hook-up, but I may in the future -500w, no more - and this is still going to require an alternator upgrade. I can't figure out why my truck is setup with a towing and offroad package (Chevy Silverado Z71 with an L31 350), but still only has a 105 amp alternator. Heck, I was surprised when the spider gears snapped, I would have thought that it would have had a posi diff. Sorry about the rambling, I had been a mechanic for 17 years before I became a miner.
I do have an Associates of Science with an emphasis on Mathematics, but I make more money/benifits working at the mine (haul truck driver) here in White Pine county, NV than I could by getting a job with the state, or a private engineering firm like I was going to do. I mention this because I can figure out everything that I need to, but I know that there are varibles that I can't account for in my equations, and in this situation only experience can help me, which I just don't have.

Basicly, I just want to get by with as little headache as possible.

Thx again.

John KC8NTP, east central Nevada

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KB3MMX
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Posts: 132




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« Reply #8 on: March 21, 2007, 12:55:47 PM »

John, to be honest..have you considered how much you are going to have to mess with the MFJ antenna if you plan on QSY'ing all the time. It kinda negates the "screwdriver" idea being that you actually have to manually raise and lower the body of the antenna and set it with a wing nut.
 The "little Tarheel" antenna is rated for 200Watts and it comes with a adjustment switch so you can tune the antenna from in the cab without all the oter garbage you would need with the MFJ product. You can also use the www.n2vz.com auto tuner like I have on my Tarheel Model 75 so you press the tune button on the radio and it does it all automatically.
 If money is tight though, I guess it means a non motorized screwdriver is the only option.

 with the MFJ Being Non-Motorized......is it technically still a "screwdriver antenna" anyway Huh

 About the rest of the setup, noise is a big issue on HF and some vehicles seem to be more prone than others and there are ways to elimninate that.
 Once again, Alan's site has some real good info on how to battle issues with a mobile setup and how to improve it's effectiveness.

 Take care and good luck,
 
               DE  KB3MMX   CHUCK
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KB3MMX
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Posts: 132




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« Reply #9 on: November 16, 2007, 09:30:31 PM »

PS> DXengineering has a nice mobile matcher unit for matching the base impedence of mobile antennas...PN# MM-1

 GL!
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