Tuning a 102"

[roadrage]

I have done tons of reading and can't seem to find an answer.
I have a 102" that I put on the roof. Coax started out 17' or 18' but I cut it when I made the inductor. It is now about as short as it took to get to the antenna from the radio (approx. 9' or 10'). Cutting the coax didn't change SWR (as some have said). The terminal connectors that attach the coax to the antenna are all soldered not crimped. It is mounted on a 6" barrel spring, and ball mount.

With the power at minimum (Galaxy 95t2) the SWR needle doesn't hardly move. Power at max (30W DK) the SWR was 1.5 on all (11M) channels. I moved the band selector and keyed up ONLY long enough to get a reading. SWR goes up the further from 27 Mhz I get, in either direction. So it is flat on 11M. Being flat means that physical lenght of the antenna is correct. I made an inductor coil and put it at the feed of the antenna and it brought me down from 1.5 to 1.3 with power at max.

I want it lower because I saved for months and bought an amp for my radio and I need to not smoke it. SWR seems to increase as power increases (as described below). Any ideas on how to get it down? I don't have an analyzer to calculate coax length, or see resonance, etc. All I have is an SWR meter.


Also, it seems that SWR increases slightly as the power increases. What would cause that? The radio was tuned on a scope, no clipping, not over modulated, no excessive output. My DK is actually 10W lower on high than the recomended 40W. As a sidenote, my SWR rose when I bonded my truck with gound strapping, why? Also started hearing added static and engine noise on my radio.
(bonding as follows- radio bracket bolted to cab floor, 2 straps from hood to cab, 2 straps from cab to frame, 2 staps from cab to bed, 2 straps from bed to frame, 1 strap from bed to tailgate, 1 strap from frame to exhaust tailpipe.)

I do not just want to get the answer, I am trying to understand the how and whys.

 

[Brake Weight]

I, too, recently purchased a 102". I screwed it in and got 2.5+ to 1 across the 11m band. No spring. Got a spring, same thing. This mount was inches from the cab of my truck. So I moved it out on the corner of my bed and got 2 to 1, with or with out spring. My Firestik wouldn't tune down below 1.9 to 1 here either. I grounded and strapped almost what you did. Made a couple of 8" jumpers and still the same. Found a spare 18' Ratshack coax in the garage and hooked it up. I got a tunable 1.2 on the Stik and 1.5-1.7 to 1 with the 102". That's with an 18', 3', and 3' sections of rg59. I can get some marine grade rg8 at work. I'll try to make and run some of that to see what happens.

As far as you jumping between bands, I don't think you can so much with the whips. Take off the spring and try it on 10m to see how the swr does. You may have to run multiple antennas to switch bands if you keep the whip.

The varying lengths of coax to change swr is just tricking the meter. Or so I've been told. But from what I've read this is ground dependent. Poor ground is effected more by this than a solid ground.

 

[The DB]

I have done tons of reading and can't seem to find an answer.
I have a 102" that I put on the roof. Coax started out 17' or 18' but I cut it when I made the inductor. It is now about as short as it took to get to the antenna from the radio (approx. 9' or 10'). Cutting the coax didn't change SWR (as some have said). The terminal connectors that attach the coax to the antenna are all soldered not crimped. It is mounted on a 6" barrel spring, and ball mount.

Cutting coax does not change the SWR of an antenna. If the antenna is anywhere close to tuned properly the SWR results will not change much simply because you changed the length of coax.

With the power at minimum (Galaxy 95t2) the SWR needle doesn't hardly move. Power at max (30W DK) the SWR was 1.5 on all (11M) channels. I moved the band selector and keyed up ONLY long enough to get a reading. SWR goes up the further from 27 Mhz I get, in either direction. So it is flat on 11M. Being flat means that physical lenght of the antenna is correct. I made an inductor coil and put it at the feed of the antenna and it brought me down from 1.5 to 1.3 with power at max.

What is this inductor coil you made? Pictures? Any which way, the CB frequencies are the low SWR frequencies and as you move away from them the SWR goes up, that is generally how it is supposed to be.

I want it lower because I saved for months and bought an amp for my radio and I need to not smoke it. SWR seems to increase as power increases (as described below). Any ideas on how to get it down? I don't have an analyzer to calculate coax length, or see resonance, etc. All I have is an SWR meter.

An antenna tuned to 1.5 to 1 SWR will not smoke you amp. If your amp smokes in this case it is not your antenna causing it. The amplifier is also the likely cause of the higher SWR when running more power. It is not unheard of for an amplifier's matching network on the antenna side to not match up perfectly to the 50 ohm impedance of the coax, thus throwing off an SWR meter.

Also, it seems that SWR increases slightly as the power increases. What would cause that? The radio was tuned on a scope, no clipping, not over modulated, no excessive output. My DK is actually 10W lower on high than the recomended 40W. As a sidenote, my SWR rose when I bonded my truck with gound strapping, why? Also started hearing added static and engine noise on my radio.
(bonding as follows- radio bracket bolted to cab floor, 2 straps from hood to cab, 2 straps from cab to frame, 2 staps from cab to bed, 2 straps from bed to frame, 1 strap from bed to tailgate, 1 strap from frame to exhaust tailpipe.)

I do not just want to get the answer, I am trying to understand the how and whys.

For one, there is more to an antenna system than the antenna itself.

What kind of coax are you running?

Your SWR can change when you bond a vehicle. This is because you are changing the counterpoise (groundplane). This also changes the resonant point of the antenna. If you take an elevated antenna with radials and change the length of the radials the SWR and resonant frequencies will change, rf bonding has a similar effect on a vehicle. On my vehicle when it was done (Ford Explorer) it lowered static quite a bit. Strange that it would raise it on yours. I did, however, use more ground straps than you, different vehicle (although maybe not that different, don't you have a Ford Ranger?).


The DB

 

[W5LZ]

It sounds like your antenna system is working as well as can be expected, there's certainly nothing wrong with a 1.3:1 or 1.5:1 SWR.
Try this just for grins. Measure that antenna from tip to the base of that ball-mount. For instance, 102" of whip, about 4" of spring, and about 4" of ball-mount. That means your antenna is 102+4+4 = 110" long. Right? So, depending on how much inductance that coil provides, and how/where it's connected, you've at the very least got into the ballpark for tuning it given how it's mounted.
If the SWR changes much (10%+/- some) then the antenna isn't necessarily out of tune, the accuracy of your meter and any calibration that's done also factors into it. It's still ballpark'ish.
When adding an amplifier to the line up then the output impedance of that amplifier has to be figured into it. The most common reason for any significant changes in SWR in such a case isn't the antenna, but the amplifier. Sorry, a no tune amplifier's output impedance is seldom 50-52 ohms. So, until you can say from measuring that output circuit's impedance, that circuit would be the first 'suspect'.
An antenna system's SWR of less than 1.5:1 shouldn't be a problem for a properly designed and constructed amplifier.
One very simple way of eliminating one of the variables in it is to run that amplifier into a 50 ohm dummy load. What does the SWR tell you in such a case? It should tell you two things. How well the amplifier's output circuit is tuned, and if the antenna system is as close as it could/should be. See how that would work?
- 'Doc

Don't expect 'numbers' except by pure accident.

 

[Brake Weight]

Would running a higher ohm capacity coax post amp bring the swr down?

 

[The DB]

Would running a higher ohm capacity coax post amp bring the swr down?

I'm guessing you are referring to a coax that can handle more power. Running a coax with more "ohm's" would effectively change the impedance of the coax and drastically affect the antenna system in a negative way.


The DB

 

[roadrage]

It sounds like your antenna system is working as well as can be expected, there's certainly nothing wrong with a 1.3:1 or 1.5:1 SWR.
Try this just for grins. Measure that antenna from tip to the base of that ball-mount. For instance, 102" of whip, about 4" of spring, and about 4" of ball-mount. That means your antenna is 102+4+4 = 110" long. Right? So, depending on how much inductance that coil provides, and how/where it's connected, you've at the very least got into the ballpark for tuning it given how it's mounted.
If the SWR changes much (10%+/- some) then the antenna isn't necessarily out of tune, the accuracy of your meter and any calibration that's done also factors into it. It's still ballpark'ish.
When adding an amplifier to the line up then the output impedance of that amplifier has to be figured into it. The most common reason for any significant changes in SWR in such a case isn't the antenna, but the amplifier. Sorry, a no tune amplifier's output impedance is seldom 50-52 ohms. So, until you can say from measuring that output circuit's impedance, that circuit would be the first 'suspect'.
An antenna system's SWR of less than 1.5:1 shouldn't be a problem for a properly designed and constructed amplifier.
One very simple way of eliminating one of the variables in it is to run that amplifier into a 50 ohm dummy load. What does the SWR tell you in such a case? It should tell you two things. How well the amplifier's output circuit is tuned, and if the antenna system is as close as it could/should be. See how that would work?
- 'Doc

Don't expect 'numbers' except by pure accident.

It is a TNT 600HD amp. I am sure, by the advertised output, it probably teeters on the edge of meltdown during normal operation. So SWR needs to be very low. I am guessing that is why it smoked. The SWR was 1.6 with the amp on, and I need to get it down. I thought that would be safe enough, though not ideal, and I was wrong. When it gets back from repair, I am also backing off the input a bit below the spec. The few less watts output won't be measureable anyway.

 

[Brake Weight]

I'm guessing you are referring to a coax that can handle more power. Running a coax with more "ohm's" would effectively change the impedance of the coax and drastically affect the antenna system in a negative way.


The DB

Right, coax to handle more power. I'm electrically illiterate. :blushing: A larger coax from the amp to the antenna is what I'm referring to.

 

[The DB]

It is a TNT 600HD amp. I am sure, by the advertised output, it probably teeters on the edge of meltdown during normal operation. So SWR needs to be very low. I am guessing that is why it smoked. The SWR was 1.6 with the amp on, and I need to get it down. I thought that would be safe enough, though not ideal, and I was wrong. When it gets back from repair, I am also backing off the input a bit below the spec. The few less watts output won't be measureable anyway.

An swr of 1.6 is not the cause of the smoke.


The db

 

[roadrage]

An swr of 1.6 is not the cause of the smoke.


The db

They seemed to think so, or are trying to cover their butts. I don't want to argue with them about it. I am hoping they fix it and I'm only out shipping cost.

 

[roadrage]

Cutting coax does not change the SWR of an antenna. If the antenna is anywhere close to tuned properly the SWR results will not change much simply because you changed the length of coax.

I have read that over and over. Still there are plenty of people that believe it does. I understand that the coax length won'd change the SWR of the antenna, I expect it can fool the amp or meter. I may be wrong, but old wives' tales tend to have a certain degree of truth. But not always.

What is this inductor coil you made? Pictures? Any which way, the CB frequencies are the low SWR frequencies and as you move away from them the SWR goes up, that is generally how it is supposed to be.

On my radio the CB freqs are in the middle. It goes from 25-28 Mhz. I just keyed up to see where I was at because from 26.965-27.405 my SWR was totally flat. I needed to get a reference of the SWR curve so I checked above and below. I guess it is good that I am at the right physical length.

I can describe it. I took apart an old alternator and took some of the stator coil winding to make my inductor. It is solid copper wire that is coated with a nonconductive material that resembles laquer. I wound it into 9 - 1" coils. I soldered two eye type terminal connectors to either end. One end was attached to the outer mounting bolts for the ball mount. The other end was attached to the center stud of the antenna. I laid the inductor coil horizontal, keeping as much space as I could between the roof, headliner, roof support, and antenna.

An antenna tuned to 1.5 to 1 SWR will not smoke you amp. If your amp smokes in this case it is not your antenna causing it. The amplifier is also the likely cause of the higher SWR when running more power. It is not unheard of for an amplifier's matching network on the antenna side to not match up perfectly to the 50 ohm impedance of the coax, thus throwing off an SWR meter.

It did smoke. I only tried it with a few short key ups through the day to try it out. On my 3 minute drive to install a roof on my church's garage, when I broke for lunch, ride home, etc. I went with my wife that evening and wanted to show her my new pride and joy in action. "what's that burning smell?" she says, as my truck fills with smoke. Very impressive {Cry_river}
I am totally new to running an amp. It is already turning into an expensive learning experience.

For one, there is more to an antenna system than the antenna itself.

What kind of coax are you running?

I took the coax from a Wilson 5000 antenna system that I am not currently using. I couldn't afford to buy any right now. I figured that the Wilson 5K advertises 5000W handling, so the coax should take what I was throwing out. My only other option was some cheap RG58 I have hooked to my dipole at the house. I didn't trust that.

I plan on buying some better coax. I have to wait and save some more. My wife let me save up for the amp that now needs repair. She doesn't want me spending money on my hobby with our house projects she wants done. I have to build a new bedroom to make room for our new baby. I wanted to buy a better SWR/power meter and brought it up last night, and she didn't seem on board.

I would like to get some LMR400 after Christmas. It is cheaper the more you buy, and I was planning on using it for my base antenna. I was going to buy a 70' length. 25-30' for the base antenna, and the rest I can use for the mobiles and jumpers. I found a place I can get LMR400 for .59 a ft.

Your SWR can change when you bond a vehicle. This is because you are changing the counterpoise (groundplane). This also changes the resonant point of the antenna. If you take an elevated antenna with radials and change the length of the radials the SWR and resonant frequencies will change, rf bonding has a similar effect on a vehicle. On my vehicle when it was done (Ford Explorer) it lowered static quite a bit. Strange that it would raise it on yours. I did, however, use more ground straps than you, different vehicle (although maybe not that different, don't you have a Ford Ranger?).

So is the antenna's resonance better with it bonded even though the SWR is higher? I am assuming so, as I have read that the point of where the antenna has the best resonance isn't always where the SWR is lowest.


I have an old F150 that I bought back in '98 that I have kept for a fishing truck. That one houses my Grant XL and has my dual antennas. I wouldn't dream of running power through that cophase set up. Some day I'd like to get a Texas Star 500 and put up a 102" on that truck. Who knows how long that'll be. When the bills are paid up and the house is done. That truck will probably be a pile of rust in the driveway by the time my wife lets me.

The truck that I am working on now, with the Galaxy 95t2/amp combo, is my 02 Chevy Silverado 2500HD.

 

[AudioShockwav]

Question.
The TnT 600HD has 4, 2 sc2879 Transistors in it, and if I remember right the Galaxy 95t2 is a high power radio running 2, 2sc2290 in the output section.
How much dead key and peak swing are you driving the amp with?
What kind of numbers are you seeing from the Amp?
And what kind of meter are you using to read power?
1.5:1 is not bad for most radio set-ups unless you are running high voltage, or pushing the transistors in the Amp hard.
X-force/maganaforce/TnT builders push users to keep reflected power as low as possible because often they know that to get the peak numbers that they advertise from there amps things are running at stressed levels.
Did they tell you what happened to the Amp?
I will say that they more often than not will fix the amp the first time ( at least that is what I have found in the past) they might or might not warranty the Transistors.
As you have described it, your antenna system is a good set-up, in fact better than the average 1/4 wave set-up in that you understand what is going on at the antenna.


73
Jeff

 

[The Jerk]

It did smoke. I only tried it with a few short key ups through the day to try it out. On my 3 minute drive to install a roof on my church's garage, when I broke for lunch, ride home, etc. I went with my wife that evening and wanted to show her my new pride and joy in action. "what's that burning smell?" she says, as my truck fills with smoke. Very impressive {Cry_river}
I am totally new to running an amp. It is already turning into an expensive learning experience.

The truck that I am working on now, with the Galaxy 95t2/amp combo, is my 02 Chevy Silverado 2500HD.

I would hazard a guess you ran too much radio into too small of amplifier...you drove a four transistor amplifier with a two transistor radio, and that's not healthy. A single 2879 can over saturate four 2879s, let alone two...

TNT600HD
Transistors 4 x 2SC2879

 

[The DB]

I have read that over and over. Still there are plenty of people that believe it does. I understand that the coax length won'd change the SWR of the antenna, I expect it can fool the amp or meter. I may be wrong, but old wives' tales tend to have a certain degree of truth. But not always.

Being that the antenna and the coax have different impedances the coax actually modifies the impedance shown. This can make the impedance appear as less or more. This does not change how the antenna functions, if anything it simply hides a potential problem or make it seem like it is not as bad, but can also make it appear worse.

On my radio the CB freqs are in the middle. It goes from 25-28 Mhz. I just keyed up to see where I was at because from 26.965-27.405 my SWR was totally flat. I needed to get a reference of the SWR curve so I checked above and below. I guess it is good that I am at the right physical length.

I can describe it. I took apart an old alternator and took some of the stator coil winding to make my inductor. It is solid copper wire that is coated with a nonconductive material that resembles laquer. I wound it into 9 - 1" coils. I soldered two eye type terminal connectors to either end. One end was attached to the outer mounting bolts for the ball mount. The other end was attached to the center stud of the antenna. I laid the inductor coil horizontal, keeping as much space as I could between the roof, headliner, roof support, and antenna.

I have a similar band setup on my radio, and it is a common system. I am running a Galaxy DX55HP, and have seen this setup used on many different radios.

As far as the coil is concerned you shouldn't need one on a 1/4 wave antenna such as the one you are running. If it is the proper length the capacitance and inductance should cancel each other. Adding an inductor of any kind will throw off this balance, as well as shorten the length that the antenna needs to be to be resonant. I am curious what an antenna analyzer would say about the resonant point of the antenna before and after this coil was added.

Also note, an SWR match does not mean an antenna is resonant. While most antennas are designed to keep these two points close, it is possible for an antenna to be resonant at a wildly different impedance than 50 ohms. Tuning to resonance will net better performance than tuning to an SWR match.

It did smoke. I only tried it with a few short key ups through the day to try it out. On my 3 minute drive to install a roof on my church's garage, when I broke for lunch, ride home, etc. I went with my wife that evening and wanted to show her my new pride and joy in action. "what's that burning smell?" she says, as my truck fills with smoke. Very impressive {Cry_river}
I am totally new to running an amp. It is already turning into an expensive learning experience.

I believe you, you should recommend it quit smoking... horrid habbit...

I see by previous posts on this and other forums that you did do research, at least on some things. That is more than many people do before running an amp. Amps can be expensive, and there is a lot of misinformation pertaining to them. They are also among the first places people go to increase their range when it should be their last. You at least did other stuff first.

I took the coax from a Wilson 5000 antenna system that I am not currently using. I couldn't afford to buy any right now. I figured that the Wilson 5K advertises 5000W handling, so the coax should take what I was throwing out. My only other option was some cheap RG58 I have hooked to my dipole at the house. I didn't trust that.

I plan on buying some better coax. I have to wait and save some more. My wife let me save up for the amp that now needs repair. She doesn't want me spending money on my hobby with our house projects she wants done. I have to build a new bedroom to make room for our new baby. I wanted to buy a better SWR/power meter and brought it up last night, and she didn't seem on board.

I would like to get some LMR400 after Christmas. It is cheaper the more you buy, and I was planning on using it for my base antenna. I was going to buy a 70' length. 25-30' for the base antenna, and the rest I can use for the mobiles and jumpers. I found a place I can get LMR400 for .59 a ft.

The rating for the Wilson 5000 is based on the coil within it, not the coax used. Look it up, you will see that it is not rated for near that much power. It should be able to handle that amp though. I wouldn't trust the RG-58 myself either, unless I happened to know it was mil-spec. Even then I just don't like RG-58. LMR-400 would be overkill for the power rating of your amp I think. That being said, go for it.

So is the antenna's resonance better with it bonded even though the SWR is higher? I am assuming so, as I have read that the point of where the antenna has the best resonance isn't always where the SWR is lowest.

Most people think that the whole car acts as counterpoise, they see the DC grounding used in most cases and think a ground is a ground. What many people don't understand is that a ground at 27.x Mhz (cb frequency range) is very different than a DC ground. For example, on my Maco V5000 base antenna there is an electrical short between both conductors of the coax when using a DC measuring tool, however at 27.x Mhz this dc ground connection acts very different, and in fact is part of the tuning mechanism of the antenna. In your car there are many DC connections between the various metal pieces, and in some cases no real connection at all. Non-conductive things like paint get in the way. What RF bonding does is make the whole of the vehicle act as one unit for the counterpoise, rather then several separate components that all happen to be together. It also gives you a better capacitive coupling between the counterpoise and the actual ground lane for the antenna, which is the ground itself.

I have an old F150 that I bought back in '98 that I have kept for a fishing truck. That one houses my Grant XL and has my dual antennas. I wouldn't dream of running power through that cophase set up. Some day I'd like to get a Texas Star 500 and put up a 102" on that truck. Who knows how long that'll be. When the bills are paid up and the house is done. That truck will probably be a pile of rust in the driveway by the time my wife lets me.

The truck that I am working on now, with the Galaxy 95t2/amp combo, is my 02 Chevy Silverado 2500HD.

Ahh yes. I have seen you on several other forums, that is where I may have picked that up from...


The DB

 

[AudioShockwav]

As far as the coil is concerned you shouldn't need one on a 1/4 wave antenna such as the one you are running. If it is the proper length the capacitance and inductance should cancel each other.

Just a quick comment, because almost every mobile antenna set-up has a different challenge often a 1/4 wave over whatever ground that a vehicle may present to the antenna you might not necessarily see a perfect 50 ohm match at resonant length of the antenna.
Sometimes it can take a small correction to get things to balance out.
I think this is what propagates the old Cb myth that you must trim the coax on a 1/4 wave antenna to make it work right.
I do think the user here has put in a diligent effort to try to understand what he is up against.
I really think he was on the right path as far as the antenna, the drive level into the amp i think is the killer here.
Simply reducing drive levels and leaving the antenna system alone should be all you need.
Cb style amps are almost always over rated and create more complex problems sometimes.

73
Jeff