| Author |
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Mikefromms
| | Posted on Tuesday, February 03, 2004 - 4:19 pm: | 
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I know a guy who has an A-1700 mobile antenna. It is very broadbanded with the radio only, but when the amp is turned on (even on low 60 watts deadkey) the swr varies from 1.5 on channel 40 to over a 3 swr on channel 1. Why would the amp make this much difference in swr?
mikefromms |
Bigbob
| | Posted on Tuesday, February 03, 2004 - 10:37 pm: |
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Is he adjusting the meter with the radio then flipping the amp on or is he re-adjusting with amp?By doing the former you will get these readings,if the meter is in the radio,then use a twelve foot jumper between radio and amp,sometimes do to cold solder joints and loose connections in the antenna you can get arcing that will raise impedance of antenna,but this is a quality product so I doubt it,the amp could be generating a high degree of harmonics and this will show as increased swr,but generally the increase would be more uniform across the band,unless the amp is old or junk,or been abused,without actually seeing the system and it's condition I can't be certain,but then you asked for a tech and I'm no tech so just read this for entertainment value and no more.Bigbob |
Kc0gxz
| | Posted on Thursday, February 05, 2004 - 12:43 am: |
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Mikefromms
Just one question. This may sound silly to you but, just where in circuit does he have his SWR meter located? In other words, is it mounted between the radio and amp or is it mounted between the amp and antenna.
I'm not trying to insult anyones intellegents, I'm just curious to know how he has this thing set up. Including coax lengths.
Jeff, kc0gxz. |
Mikefromms
| | Posted on Friday, February 06, 2004 - 5:59 pm: |
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Yes, the swr meter on the radio reads very low swr. The Rad Shack meter with 3 foot jumper between the antenna and the amp reads high. Note he is using a magnetic mount.
mikefromms |
Kc0gxz
| | Posted on Friday, February 06, 2004 - 11:32 pm: |
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Mikefromms
A power amplifier going into a mag mount is not a good idea. This arrangement makes for a very poor grounded system and isn't the best set-up for use with a RF power amplifier. It's not that it won't work, it just won't work to its best capabilities.
Whenever you are playing with RF Amps, always have EVERYTHING properly grounded. ESPECIALLY the antenna since this is the heart of the whole system. Mag mounted antennas ARE NOT truely grounded.
Also, I'm very skeptical when it comes to Radio Shack antennas, meters and especially their coax and jumpers.
Show him how to do it Mike. I know you know how.
Jeff, kc0gxz. |
Mikefromms
| | Posted on Saturday, February 07, 2004 - 9:13 am: |
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I suspected the mag mount and doubted the accuracy of the rad shack meter.
If you didn't want drill into your rooftop, which would you do Jeff, drill and mount antenna on the trunk or mount it to the bumper? I know the antenna would be maybe 3' higher on the trunk but do you think the bumper mount would make the better ground connection to the body? Just your thoughts.
mikefromms |
Kc0gxz
| | Posted on Saturday, February 07, 2004 - 3:15 pm: |
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Hi Mike
Was it mine, I'd drill into the trunk. A bumper mount will give you zero groundplane off the rear of the car.
Another way he could do this without any drilling is to go with a trunk-lip mount. They're cheap, easy to install, no holes to drill and provide a better ground, not to mention the fact that he would have some groundplane off the rear.
Doing this is no guarentee that he will have a better SWR at the output of his amp, but at least he will have a grounded system. There's no point in having the radio and amplifier grounded and not the antenna system.
Let me know what your end results are. And what ever you do, don't take the SWR readings as gosple from his Radio Shack meter. Double check the SWR with another brand of SWR meter. I'm curious to know what you come up with and how everything works out.
73s for now Mike.
Jeff, kc0gxz. |
Mikefromms
| | Posted on Monday, February 09, 2004 - 10:19 am: |
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Thanks Jeff, you got me thinking. I'll let you know what happens. I'll be sure to use another meter as well.
mikefromms |
Ironmask
| | Posted on Monday, February 09, 2004 - 2:00 pm: |
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Mike,
With your permission I would like to add my 1 1/2 cents worth.
What type of vehicle are we discussing? A van, car or pick-up? Maybe I miss something there.
There is also a gutter mount made that I would mount on the passenger side of the vehicle at the closest to the middle [front to back] of the vehicle. This would permit a good ground plane toward the opposing traffic as well as front and back. It would not function as good as a lip-mount, but whether the lip mount is mounted on the trunk or hood there is still a problem when either is raised. Once the gutter mount and coax are installed you wouldn't have to worry about it. The other plus is the antenna would be that much higher.
When I said as good as a lip-mount I was reffering to it's ability of mounting in the center, width wise.
In past instalations with a multi- magnet set up I have used a 10 gauge wire souldered to a 3" dia. 1/16' piece of tin. I placed that undr one of the magnets. I used saran wrap between roof and magnet. Then I used a small bead of di-electric grease around the magnet where it attaches to the roof.
I must also agree that any mount other than a magnet has better ground qualities.
That' just my 1 1/2 cents.
Iron Mask |
Bigbob
| | Posted on Monday, February 09, 2004 - 7:23 pm: |
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8gauge wire with a q-d and one end attached with a no.12stainless sheetmetal screw and Gun-grease gobbed on it in the top of the vehicle,the other end attached to the grounded end of antenna,works for me. |
Mikefromms
| | Posted on Tuesday, February 10, 2004 - 12:03 pm: |
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Ok, here's the plan: An Astatic 600 meter, trunk lip mount and low pass filter has been ordered. The amp will be grounded with 213 coax shielding.
This covers all of the suggestions made thus far. We'll let you know how it works out.
mikefromms |
Mikefromms
| | Posted on Tuesday, February 10, 2004 - 3:43 pm: |
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Oh I will replace the rad shack coax that came with the rad shack magnet when I get my lip mount in. Wouldn't be surprised if the coax and magnet are the culprits here.
Anyway, thanks for all the help.
mikefromms |
Kc0gxz
| | Posted on Thursday, February 12, 2004 - 1:56 am: |
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Mikefromms
What??
Lol. OK, I give up Mike. I just have to ask.
How-come-for-why-gee-wiz are you running a Low-Pass filter in the car? Am I missing something here?
Jeff, kc0gxz. |
Tech833
| | Posted on Friday, February 13, 2004 - 8:52 pm: |
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How come he is running an amp with a magnet mount antenna anyway?
The 'ground' connection (let's call it the counterpoise) is capacitive when using a magnet mount. Look at is as a capacitor between the coax shield and the counterpoise system, which makes up 1/2 the antenna system. Capacitors in the shield are a little O.K. at high frequencies, but worthless on DC. So when the capacitor becomes saturated (like when running higher current with an amp), its value decreases. When the value decreases, it is no longer any good at HF either. It is essentially an 'open' connection.
You don't want to see what happens to the finish under the magnet mount after running a setup like that for a while either. A hole is much less ugly in the long run. |
Mikefromms
| | Posted on Sunday, February 15, 2004 - 10:47 pm: |
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Hello...I'm back. Took the low pass filter off (forgot where I learned that one) trunk mount didn't work because it would cause a leak (gapped open my rubber seal). I did use my new meter to tune the radio as recommend by the tech who did the review on the KL-300. Also, checked swr with new meter.--no problem with amp on or off. I guess I need to just drill that hole and be done with it. I'm sure the owner won't mind.
mikefromms |
Mikefromms
| | Posted on Monday, February 16, 2004 - 6:07 pm: |
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We'll drill the hole on Tuesday and replace the coax and be done with it all. The car has 211,000 miles on it so it's not like I'm really hurting the "resale" value, huh? I plan on driving it another year or two.
mikefromms |
Therealporkchop
Junior Member
Username: Therealporkchop
Post Number: 36
Registered: 11-2002
| | Posted on Sunday, June 06, 2004 - 2:37 pm: |
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I have one of those Rad Shack meters too. I have a few others as well. A diamond, a gold line, a aries and a dosy. My Rad Shack meter is close, so it's not that bad. I have the same problem though, running a mag mount with a linear. Linear off, good swr, linear on, bad swr. Antenna toward the bed of my dumptruck, swr gets better, toward the front of truck, swr goes up higher. I'd like to use the roof mount, but I hate to drill a hole in a brand new truck. Especially since using my mag mount in the center gives a bad swr. Hate that mag mount hitting trees and flying all over my roof though.
The low pass filter thing is this: the amp is causing extra frequencies in the second harmonic and since it isn't in the range you are on, it's feeding back down the line (or something like that, get the terms confussed sometimes like Tim Allen). The low pass helps cut that out somehow. I read this in a book somewhere, I think Lou Franklin wrote it, don't really remember. I'm thinking of trying it myself. If I don't go roof mount, I'm going to try the tin with a ground wire attached run to the chassis... |
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