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What brand of screwdriver antenna is this
- Thread starter KMJensen
- Start date
So their is an insulator... Odd that you are getting a resistance instead of an open or short circuit. If it were an open circuit I would say it was a capacitor and part of a matching system. Their aren't very many reasons that you would want to have DC resistance on an antenna as DC resistance translates directly to heat loss. I wonder if their is something else going on...
Looking at the image it looks like their is a screw that makes it to the center part of the tube. Does that screw make it all the way to the outside? Looking at the provided images above it appears to go all of the way out, and it appears their are 2 or 3 that do the same. That being the case I'm surprised that your multi-meter isn't simply showing a short circuit, unless perhaps said screws weren't tightened down all the way when the reading was taken?
The DB
You are correct. The screws were not fully tightened. If they were, they would result in a complete short. The more I delve into this the more I learn and the more questions arise. The three lower screws appear to ground the matching coil. Shouldn't the body of the antenna be isolated frm the ground?
When it comes to RF things get far more complicated than DC. The inductor, which at DC appears to be a dead short, can appear to be any number of things to an RF circuit, and if designed a certain way can even appear to be almost an open circuit. This is how many RF chokes are made, and will allow DC to flow freely but block RF of certain frequencies. This is a more extreme case, but just because you have a DC short does not mean it acts the same at RF. The shunt coil mentioned above also presents a DC short, but all it does at RF frequencies is modify the impedance between the feed line and the antenna as a means of matching.
The DB
Not sure why that insulator would be there except possibly a bushing to fill the gap.
Yes the entire antenna has to be isolated from ground. The matching coil must short to ground though but will have no adverse effect on the antenna. That's why I posted earlier a lead, preferably a piece of enameled wire would have to come off the coil and go to ground.
After looking at those pics some more, I don't see how the coil would work because the entire coil isn't isolated and both tap point screw touch the antenna body too. If that lower section was isolated from the rest of the antenna then possibly.
I would bypass it and go with a 9 turn shunt coil and be done with it.
That antenna appears to been mounted using a stud mount and ring terminal for the coax. You can use this type or the typical SO 239 stud type. Both of these use nylon insolators to keep from shorting the antenna out.
Sere pics below.
SO 239 stud mount with white nylon insolator.
Ring terminal with 2 insolators that go between the mounting bracket. 3/8 x 24 bolt to go into antenna base. This is what your antenna previously used except he top nut wasn't.
Typical mounting configuration so you can see how the antenna is isolated. You can use either type to mount your antenna. You can use one like this with or with the top nut. You will need a 1 inch piece of all 3/8 x 24 all-thread to use the top nut though. Most CB dealers sell these.
Yes the entire antenna has to be isolated from ground. The matching coil must short to ground though but will have no adverse effect on the antenna. That's why I posted earlier a lead, preferably a piece of enameled wire would have to come off the coil and go to ground.
After looking at those pics some more, I don't see how the coil would work because the entire coil isn't isolated and both tap point screw touch the antenna body too. If that lower section was isolated from the rest of the antenna then possibly.
I would bypass it and go with a 9 turn shunt coil and be done with it.
That antenna appears to been mounted using a stud mount and ring terminal for the coax. You can use this type or the typical SO 239 stud type. Both of these use nylon insolators to keep from shorting the antenna out.
Sere pics below.
SO 239 stud mount with white nylon insolator.
Ring terminal with 2 insolators that go between the mounting bracket. 3/8 x 24 bolt to go into antenna base. This is what your antenna previously used except he top nut wasn't.
Typical mounting configuration so you can see how the antenna is isolated. You can use either type to mount your antenna. You can use one like this with or with the top nut. You will need a 1 inch piece of all 3/8 x 24 all-thread to use the top nut though. Most CB dealers sell these.
if it works as i imagine you need the insulator to support and insulate the coil,
imagine the shunt coil setup been on a former stuck into the bottom of the antenna rather than dangling in free space,
then imagine taking the coil away and cutting the 9 turn coil into the bottom of the tube shoving the insulator up inside to support it
the screws short the coil to ground for bands where 9 turns is too much,
feed it at the solder tags.
imagine the shunt coil setup been on a former stuck into the bottom of the antenna rather than dangling in free space,
then imagine taking the coil away and cutting the 9 turn coil into the bottom of the tube shoving the insulator up inside to support it
the screws short the coil to ground for bands where 9 turns is too much,
feed it at the solder tags.
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Yes and no. On lower bands like 80 meters, the coil may have to be re-adjusted or more turns added because the feed point impedance lowers even more. You can adjust the coil for a good balance between 80 and 40 and probably 20 but then 17 meters and up will have to be re-adjusted. This is the trade-off for running 80 meters on a shortened mobile antenna. 10 thru 40 will average out just fine with a single shunt coil.
I've got away from shunt coils and moved on to dual UNUN transformers that I modified with a toggle switch for easy switching between the upper and lower bands. I'm going to modify it more with a relay switch so I can remote switch it from in the vehicle.
Your antenna has the same coil diameter and about the same coil length as my screwdriver antenna so I can say yours will only tune down to 40 meters. 80 meters on a mobile is about as in-efficient as you can get and 160 meters is even worse. The efficiency rating on an average mobile antenna for 80 meters is less than 2 percent. So with a 100 watt rig, your ERP is barely 2 watts. The 160 meter ERP is in milliwatts! There are ways to get the efficiency increased on 80 meters with a screwdriver antenna but it's only for stationary operating. You can inquire about that later on if and when you get this antenna going.
That original matching coil design is a bit baffling. If we saw a complete version of it we would most likely be able to figure it out. To me it seems that one end of the coil be connected to the antenna body the rest of the coil would have to be isolated from the antenna body, then have specific tap points on the coil and be shorted to ground to work properly. If the coil wire was enameled wire, those tap screws wouldn't make good contact. If it was all bare wire then the coil would be touching the antenna body and those tap screws wouldn't make much difference.
I know this is a old thread but it looks like an early Alpine Antenna. Wes made his just like that with the stud and the coil turns on the base so no shunt outboard was needed. I had a couple of them. He is silent key now.
