prcguy
Member
How did you mount the antenna? I kind of looks like its on a bracket sticking out the side of the bulkhead rack thing.
3 questions. Using pass through on cab and RF Grounding??
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Yes it's a chunk of 3/8 X 4" plate that I cut a 2" X 2" square off the front to wrap around the post and weld on.
Then where the antenna sits I just happen to have a 1/2" thick by 3" diameter machined washer to weld on so it has a nice flat surface to sit on since 3/8 strap is never flat.
I can lift the bed off with this mount so it should be fine.
Now I need to wind the chokes for the power and reed switch.
I was thinking of going higher but I don't want to be stopped by a truck cop for going over the height limit. It's pretty darn tall now.
prcguy
Member
I thought we mentioned this before, but the antenna feed point wants to be up against a large area of ground plane, or flat sheet metal. When you raise the feedpoint or move it away from the ground plane it raises the feedpoint impedance, screws things up and performance will suffer. A classic example is mounting a screwdriver antenna to a trailer hitch that sticks out away from the body. What you made is about the same thing. If you could move the antenna so it mounts right on the bed of the truck it will work much better even though its lower.
I think what the factory told you is mostly BS as the majority of radiation off their antenna is below the coil. Or in other words, the coil and top whip do not radiate as much as the mast below the coil. If you probe around the antenna with a fluorescent light tube you will find it will light up more around the coil and top whip but that's because the voltage is higher there in the near field. Maximum radiation is actually in the area with the most current, which is the lower part of the antenna starting at the feedpoint.
A typical large screwdriver installed properly should have a feedpoint impedance on 40-80m in the 5-15 ohm range. When you mount it to a trailer hitch it might have a great match on 40-80m closer to 50 ohms without any shunt coil and that means you have introduced 20-40 ohms of loss in your antenna system by moving the feedpoint away from the ground plane.
Your antenna will tune and make contacts where you are mounting it but you might have neutered it to the point where it probably radiates like a cheap Hamstick that was mounted properly. You spent good $$ on that antenna and you should install it right so you get your $$ back out of it.
I think what the factory told you is mostly BS as the majority of radiation off their antenna is below the coil. Or in other words, the coil and top whip do not radiate as much as the mast below the coil. If you probe around the antenna with a fluorescent light tube you will find it will light up more around the coil and top whip but that's because the voltage is higher there in the near field. Maximum radiation is actually in the area with the most current, which is the lower part of the antenna starting at the feedpoint.
A typical large screwdriver installed properly should have a feedpoint impedance on 40-80m in the 5-15 ohm range. When you mount it to a trailer hitch it might have a great match on 40-80m closer to 50 ohms without any shunt coil and that means you have introduced 20-40 ohms of loss in your antenna system by moving the feedpoint away from the ground plane.
Your antenna will tune and make contacts where you are mounting it but you might have neutered it to the point where it probably radiates like a cheap Hamstick that was mounted properly. You spent good $$ on that antenna and you should install it right so you get your $$ back out of it.
Yes we did talk about this. I even have holes in the truck bed. But after calling the company and talking with the owner and listening to what he and other had tested over the years it made sense. I can always drop it back down. He said and I quote. The reason the antenna does poorly when mounted to a hitch is because RF balloon is forced into the tail gate.
I guess they have done extensive testing. I know I would if this was all I built.
I need to find someone in my area that can teach me and has some experience. I have always come from the world of AM and CB where yes that metal is what makes it go. But this is a hole other world so all I can do is take the advice of others. I found a club near by so I will stop over there and see if anyone can help or has a tuner. I’m in over my head a bit on this build but I’m sure it will work.
I have always gone by the rule of, Ground plane Ground plane Ground plane, Ground plane,
To me this seems totally anti normal to lift the antenna like this.
But then again I can only go off of what others have done.
I guess I will find out.
Another thing he was telling me is to use solid copper for making the ground to the truck.
He was saying in testing they have found that the amount of surface area make a difference. 1 inch of surface area is the minimum you want.
I guess they have done extensive testing. I know I would if this was all I built.
I need to find someone in my area that can teach me and has some experience. I have always come from the world of AM and CB where yes that metal is what makes it go. But this is a hole other world so all I can do is take the advice of others. I found a club near by so I will stop over there and see if anyone can help or has a tuner. I’m in over my head a bit on this build but I’m sure it will work.
I have always gone by the rule of, Ground plane Ground plane Ground plane, Ground plane,
To me this seems totally anti normal to lift the antenna like this.
But then again I can only go off of what others have done.
I guess I will find out.
Another thing he was telling me is to use solid copper for making the ground to the truck.
He was saying in testing they have found that the amount of surface area make a difference. 1 inch of surface area is the minimum you want.
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As he explained to me.
This section of the antenna as seen in the picture,
Must be free and above the tallest point of all metal by at least 12 inches when in the transport position no matter where it is mounted.
The higher the better.
Him and some other big contest guys have done extensive testing. The guy he was referring to is back east and has some 300 foot towers on his land. I ran across his web page and his articles and quickly realized he was way over my pay grade.
Now as he went on he also explained about how they were grounding the trucks and bodies. Ground everything block, transmission, exhaust, hood, everything. Grind down the paint make the bond and seal up the area with clear to stop rust.
I take everyone's input and try to apply it where I can. Like I said, I'm way in over my head. My last radio was an old Heathkit SSB and a long wire. Super simple and forgiving. But these new radios are not as forgiving.
Once it stops raining and I make progress I'll snap you some more pictures for input.
Thank you for the help so far.
Once I get it working and find a ham in the area that is willing to give me a hand then we can actually start to see what is going on.
K6OEF
This section of the antenna as seen in the picture,
Must be free and above the tallest point of all metal by at least 12 inches when in the transport position no matter where it is mounted.
The higher the better.
Him and some other big contest guys have done extensive testing. The guy he was referring to is back east and has some 300 foot towers on his land. I ran across his web page and his articles and quickly realized he was way over my pay grade.
Now as he went on he also explained about how they were grounding the trucks and bodies. Ground everything block, transmission, exhaust, hood, everything. Grind down the paint make the bond and seal up the area with clear to stop rust.
I take everyone's input and try to apply it where I can. Like I said, I'm way in over my head. My last radio was an old Heathkit SSB and a long wire. Super simple and forgiving. But these new radios are not as forgiving.
Once it stops raining and I make progress I'll snap you some more pictures for input.
Thank you for the help so far.
Once I get it working and find a ham in the area that is willing to give me a hand then we can actually start to see what is going on.
K6OEF
The chokes are as close to the antenna as I can get them.
I do understand that there is no way to keep water out of them so they do have breathing holes at the bottom. I at least want to keep as much water out as I can but still allow them to drain and dry out.
I do understand that there is no way to keep water out of them so they do have breathing holes at the bottom. I at least want to keep as much water out as I can but still allow them to drain and dry out.
First contact made. Sacramento to San Diego 500 miles. W6IIS returned the call. Nothing like giving a 60+ year veteran old Ham operator the honers. Conditions were bad in my location. Signal fades in and out, sitting almost directly under the power lines. But it worked. Now to fine tune everything and strap the heck out of the truck. Need more power now. LOL
The final configuration with the ground strapping done.
If you are wondering about the straps. There made by Empi. They are VW ground straps. You can get them off Amazon for around 7 bucks.
I have gone through the hole truck and strapped everything. Antenna to bed. Bed to frame. Frame to cab. Cab to fenders. Cab to hood and anything else that is there.
Second contact was New Mexico. 1500 miles. Not bad for a mobile.
To keep the straps from oxidizing I soaked them with clear lacquer. That also makes them keep there shape.
Now to finish up all the little odds and ends. Then it's time to start looking at setting an amp in the system.
The KPA500 looks like it will work with the inverter I have in there now.
I would do the 1500 but it needs 220 @ 20 amps.
If you are wondering about the straps. There made by Empi. They are VW ground straps. You can get them off Amazon for around 7 bucks.
I have gone through the hole truck and strapped everything. Antenna to bed. Bed to frame. Frame to cab. Cab to fenders. Cab to hood and anything else that is there.
Second contact was New Mexico. 1500 miles. Not bad for a mobile.
To keep the straps from oxidizing I soaked them with clear lacquer. That also makes them keep there shape.
Now to finish up all the little odds and ends. Then it's time to start looking at setting an amp in the system.
The KPA500 looks like it will work with the inverter I have in there now.
I would do the 1500 but it needs 220 @ 20 amps.
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So the system is done but a strange thing happened after taking off the test ground straps and adding some from the cab to the frame.
The SWR went from 1:1 to over 5:1.
So I removed the straps from the cab to frame. SWR goes down from 5:1 to 3:1. OK. That was an improvement.
Remove the straps from the bed to frame. No change.
Then I swap out the nice big flat straps from the antenna with 6 small 1/2 inch straps and now it’s back to 1:1. Strange. And nobody can understand why it happened.
The SWR went from 1:1 to over 5:1.
So I removed the straps from the cab to frame. SWR goes down from 5:1 to 3:1. OK. That was an improvement.
Remove the straps from the bed to frame. No change.
Then I swap out the nice big flat straps from the antenna with 6 small 1/2 inch straps and now it’s back to 1:1. Strange. And nobody can understand why it happened.
prcguy
Member
What may be happening is that antenna on a really good ground plane will be very low impedance on the lower bands, maybe 10 ohms or less on 80m, 10-15 ohms on 40m, etc. When the ground plane is less adequate or you raise the feed point above the ground plane, the feed point impedance goes up proportionally with the losses and appears to be a better match, but its really losses creeping in. Nearly all big screwdriver antennas need a matching circuit at the base like a shunt coil that is in effect on the lower bands but out of the circuit on the higher bands. Do you have a shunt coil or matching transformer on this antenna?
Shunt coils hare nice because you can tune them to split the difference between 80 and 40m, then they don't affect much at higher freqs where its not needed. I use a 50 to 12.5 ohm matching transformer on my Tarheel but I only use 80 and 40m. It takes me from about a 2 to 3:1 match down to perfect on 80 and 40m. There should have been instructions with your antenna on using a shunt coil or transformer and I think that may be your problem. You are actually improving efficiency with the bigger straps but need to compensate with proper matching.
Shunt coils hare nice because you can tune them to split the difference between 80 and 40m, then they don't affect much at higher freqs where its not needed. I use a 50 to 12.5 ohm matching transformer on my Tarheel but I only use 80 and 40m. It takes me from about a 2 to 3:1 match down to perfect on 80 and 40m. There should have been instructions with your antenna on using a shunt coil or transformer and I think that may be your problem. You are actually improving efficiency with the bigger straps but need to compensate with proper matching.
prcguy
Member
The shunt coil will change value depending on mounting location, ground plane, etc. They must be tuned for each setup. You can spread and squeeze the coil somewhat to change its value and for a radical change in ground plane you might have to add or remove a turn, etc.
It’s sitting at 1:1 on 40m the way it’s at. But change around the ground and it goes up. I know these HF mobiles are a trade off from band to band. I’m just puzzled why adding ground from the frame to the cab would make things go up by 3. It just defying the rule of thumb on ground. That’s why I think it might have made a ground loop.
To bad your not close to sac. You would probably enjoy trying to analyze this one. It drove be nuts for a couple of days trying to figure out what I did and how to un-do it
To bad your not close to sac. You would probably enjoy trying to analyze this one. It drove be nuts for a couple of days trying to figure out what I did and how to un-do it
prcguy
Member
I'm usually a good 6hrs from you but this week I'm working on a big antenna in the north shore of Oahu. The one here is not cooperating and it will be at least a few days before I can beat it into submission.
Have you thought about mounting that antenna on the front bumper. Don't get too caught up in the "perfect" or "best" place, its more of a practical thingy...
I can tell you that the Scorpion antenna is not a light weight thingy, so count on a very "robust" mount., very firmly attached to that truck. The sheet metal type mounts are NOT going to live long or prosper...
Kinda late for this but hopefully no too late...
I can tell you that the Scorpion antenna is not a light weight thingy, so count on a very "robust" mount., very firmly attached to that truck. The sheet metal type mounts are NOT going to live long or prosper...
Kinda late for this but hopefully no too late...
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