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1-8 of 8 messages
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Strange SWR issue
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by KG9JZ on September 15, 2008
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Having a strange (to me) issue with SWR on an antenna on my mobile setup.
After countless hours looking online, I have used this as my setup
I have an IC-706 running into an LDG At-7000 tuner. Out from the tuner, running RG-8U into the antenna (102" stainess whip with a 5 inch coil. I dont have one of them screwdriver type antennas since it is out of the budget.) The antenna is mounted on the left rear bumper, think about 2 to 2 1/2 feet is how much of it sits behind the tailgate. Only have about 6" from the antenna and the tailgate at that point. I just have the center conductor hooked up (as online this is the only information I could find). I have a ground running from the tuner to the chassis inside the truck and verified the ground is working. Tried to make a contact on 40 and 75 meters at 100 watts but couldnt get across. Couldnt try 20 since it was on its way to sunset and not much activity could be heard.
The antenna is insulated from the metal chassis and receives nice and strong. It can easily be tuned from 10 meters down to 75 meters, but off the tuner I dont get under 1.5 SWR on 10/12 meters anymore. My old setup was that I was using RG-58 and I was running from a coax switch to the antenna, where the center conductor was connected and also the braid on the coax was connected to the bumper (made ground). This setup I could have an average of 1.5 SWR running 100 watts without the use of the tuner, but had trouble tuning 40 meters and half the time could tune 75 meters using 5 watts only.
Is there something I am missing somewhere? Seems like there is but I need some expert opinion on it. I dont have a hamstick to try yet but did make some grounded mounts at the top of the bed in the truck. Only problem is that if the hamsticks are about 8ft long, the tips will be toping out at over 11 1/2 feet high. At that height, I get a little paranoid around electric lines driving through town.
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RE: Strange SWR issue
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by KG9JZ on September 15, 2008
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Forgot to add, the coax switch is there because I have a mag mount quarter wave 6 meter antenna on the top of the cab. I switch to that one every now and then to see if 6 meters is up. Works pretty good too when the conditions are good
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RE: Strange SWR issue
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by K0BG on September 16, 2008
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Rather than explain everything here, I suggest you go to my web page and read the Auto-Coupler article.
Using the AT7000 to tune the whip through coax results in almost 100% loss in signal, irrespective of the SWR reading. And... the AT7000 does not have the matching range required.
As for the coil, make sure you read that part in the article.
Alan, KØBG
www.k0bg.com
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RE: Strange SWR issue
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by KG9JZ on September 16, 2008
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Oops! Seen coax is a big NO-NO for the setup. I'll try the wire route tomorrow evening when I have time and see what it brings.
Thanks for the webpage
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RE: Strange SWR issue
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by AA4PB on September 17, 2008
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Also note that the wire between the antenna whip and the tuner needs to be quite short as it becomes part of the antenna and it is radiating RF. If you lay that wire along the chassis then you will have a lot of capacitance shunting the tuner output to ground. You also need a short bonding strap between the tuner ground and the chassis.
A whip and an auto tuner can work quite well **IF** you follow all the proper installation rules, which often is not easy to do.
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RE: Strange SWR issue
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by AA4PB on September 17, 2008
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I'm also not sure the AT7000 is going to have enough range to tune a 102-inch whip on all bands even if it is properly installed. You may get it to work on 10/15/20 meters but probably not very well on 75/40M.
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RE: Strange SWR issue
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by W3LK on September 17, 2008
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You want an antenna coupler designed to feed a wire antenna, such as the ones from SGC, Alinco, Icom.
The LDG is designed for a coax feed, not a single wire.
73,
Lon - W3LK
Naugatuck, Connecticut
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RE: Strange SWR issue
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by KG9JZ on September 17, 2008
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Today I have decided that I will pick up a 20 meter and a 40 meter hamstick for their respective bands. I have mounts in the middle of the bed of the truck along the edge. The bed/body/cab are all grounded together when I first started on the project a week ago.
There is a short grounding strap between the tuner and the ground, that was put in monday. What I actually did for wire to feed the antenna was take a section of thick RG-8 coax and peel away the black casing and the inner braid, leaving only the center conductor surrounded by the 5/16 (or whatever size it is) foam insulation and ran that to the whip. Into the antenna coupler, it is fed into the center pin using a barrel wire connector that holds very tight. It doesnt work on 40 meters, but the short while I was on 20 meters this morning and early evening I had a contact right away in New Hampshire and this evening had another up in 7 land. Signal reports was good from both of them. But like above, this is just a temp setup as a hamstick will probabally be the better way to go. Think I'll coax-feed the whip and have it strictly on 10/12 meters since it runs around 1.5:1 without a use for a tuner at 100 watts.
At this current setup, its impossible to mount the tuner closer without a waterproof enclosure. But also since the truck is parked outside at night, leaving a tuner out in the open like that isnt an option at all.
But im getting there slowly and surely. Just takes some more work and sweat.
I do appreciate all the replies though, they are all helpful in the setup
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