Trying to learn why my antenna is resonant at 111"...

[62Scout]

I recently put a 102" whip on the back of my Suzuki on the spare tire mount, and in order to make it resonant on my intended frequency of 27.005/CH4, I ended up having to put a 9" spacer under it. I'm using an unmodified (as far as I know, anyways) Cobra 19 DXIII radio, connected by Laird Teflex cable (RG-58A/U Teflon), and soldered PL-259 connectors.

I made the spacer from a chunk of threaded rod and a coupler. I know that the 102" is intended to have the 6" spring under it to make it 108", but seeing as how 27.005 is virtually the only channel I ever use on CB, I felt it better to tune it to match that frequency (the CB is only used for 4x4 trail communication for me. 99% of the time we're using CH4, and only switch off in case of heavy traffic or interference). According to the antenna calculator I used, that put the antenna length at 104" for a 1/4 wave.

I used the typical 3/8-24 to SO-239 stud mount to a plate I welded to the spare tire carrier on the tailgate, behind the tire. The carrier is steel, and it attaches to a steel tail gate. I sanded down the paint on the carrier and the coating on the bolts to ensure a good ground connection. I also added a braid strap between the tail gate hinge bolt and body side hinge bolt. I did NOT install the spring, as when testing antenna flex, I found it could smack the tail gate glass and potentially break it, so I made the spacer the entire length I needed.

I used an MFJ-225 antenna analyzer for tuning, and managed to get a wide band 1.4SWR with the antenna resonant at 27.005. Testing the radio out in the deserts outside of Phoenix here with a few other guys on a trail run, I found the performance to be excellent, well beyond my expectations.

This is where I get confused. I know that the spare tire carrier is a less than ideal mounting location. I know that it should ideally be in the center of the roof, but the steel in these Suzukis is paper thin to begin with, and as I said earlier - it's only used for trail comms, so I'm OK with accepting the compromises of my mounting location. I know this location affects the radiation pattern, and I expected it to affect the SWR, but having to make it 7" longer than it _should_ have been to be resonant on 27.005 is something I just don't understand.

I'm not asking for help in making my setup "right", as dang it, it works pretty darn well as is Rather more so that I'm trying to understand WHY this is happening (I previously made the mistake of asking on a local ham radio board, and basically got told that I don't know what I'm doing, and I should just take it to a professional and pay that pro to install it...duh! OBVIOUSLY there's something I don't know here, hence why I'm asking about it! Paying someone else doesn't help me learn anything :facepalm From a performance standpoint, I'm 100% happy with this setup, and don't feel the overwhelming need to change it. But I still want to get a better understanding on why it needed to be 7" longer than what it should have been according to the antenna calculators to be resonant.

 

[prcguy]

Usually things that are spaced out and away from the vehicle sheet metal body is a very poor ground plane for antennas, even if it shows grounded with an ohm meter. What happens is you loose some capacitance from the whip to the vehicle and that can raise the impedance and/or resonant frequency of the whip.

By lengthening the whip you have added a little more capacitance between the whip and vehicle body, lowering the impedance and the resonant frequency of the whip. At some point the whip can be a little out of resonance due to the extra length but it will match better because of the additional capacitance.

A way around this is to fool the antenna by adding some capacitance at the base right at the feed point. Usually about 30pf will do but some value between 20pf and 100pf is probably needed to match the whip at its original length. A dipped silver mica cap is probably the best and for stock CB power a 50v cap is fine and go bigger if your running any power.
prcguy

 

[Project25_MASTR]

Velocity factor could also come into play.

Sounds like a tin-top samurai…have you checked out NMO low band antennas?

 

[62Scout]

Usually things that are spaced out and away from the vehicle sheet metal body is a very poor ground plane for antennas, even if it shows grounded with an ohm meter. What happens is you loose some capacitance from the whip to the vehicle and that can raise the impedance and/or resonant frequency of the whip.

By lengthening the whip you have added a little more capacitance between the whip and vehicle body, lowering the impedance and the resonant frequency of the whip. At some point the whip can be a little out of resonance due to the extra length but it will match better because of the additional capacitance.

A way around this is to fool the antenna by adding some capacitance at the base right at the feed point. Usually about 30pf will do but some value between 20pf and 100pf is probably needed to match the whip at its original length. A dipped silver mica cap is probably the best and for stock CB power a 50v cap is fine and go bigger if your running any power.
prcguy

THANK YOU!! I'll dig out some caps from the parts bin, and start playing with that as well. Looks like I also should go through the manual for my analyzer and get a better understanding of how it displays information, lol.

Velocity factor could also come into play.

Sounds like a tin-top samurai…have you checked out NMO low band antennas?

4 door Vitara...similar to the 2nd gen Chevy Tracker. Breifly considered the NMO27, but I figured the longer length of the 102 would be more beneficial to me, and a whole lot more durable given the environment that it will be in. Plus, as I said, the current set up in working quite well. If it was purely performance that I was after, I'd say it's working plenty good enough, leave it alone and move on. But it's the understanding of what's happening that I'm after here. For me, I like to know not just how to set it up to work, but also the why something works, or behaves differently than expected.

 

[kc8kek]

i think prcguy nailed it.

 

[Project25_MASTR]

THANK YOU!! I'll dig out some caps from the parts bin, and start playing with that as well. Looks like I also should go through the manual for my analyzer and get a better understanding of how it displays information, lol.



4 door Vitara...similar to the 2nd gen Chevy Tracker. Breifly considered the NMO27, but I figured the longer length of the 102 would be more beneficial to me, and a whole lot more durable given the environment that it will be in. Plus, as I said, the current set up in working quite well. If it was purely performance that I was after, I'd say it's working plenty good enough, leave it alone and move on. But it's the understanding of what's happening that I'm after here. For me, I like to know not just how to set it up to work, but also the why something works, or behaves differently than expected.

I had a Samurai for little while...ran fine in Texas but I (nor the mechanics) could never get it to run right above 9000 feet. Ended up selling it to a guy down in Galveston who had his previous one wrapped around his pier and beam in the surge Ike brought with it.

I bought an XJ to replace it. I've noticed a lot of those little Vitara, Trackers and what not are a little more popular at high altitudes due to the fuel injection. I've never really messed with the roof on a Vitara but the tin top Samurai had a paper thin roof.

 

[SOFA_KING]

Inadequate ground plane (counter poise) would require a longer whip. It's basically the off-center fed concept. Installing braided ground straps to all metal parts on the body and exhaust system (especially your mounting base) will give you a better counter poise and lower your engine noise. Also, some lengths of coax can totally detune a low band antenna. Consider cutting your coax length to odd multiples of the desired frequency wavelength (1/4, 3/4...) by (times) the volicity factor of the cable. This will ensure no nulls within the coax. Even multiples can cancel out the desired frequency. I have see it many times, and you will get maximum bandwidth from the center frequency using odd multiples.

Low band(s) in a mobile installation requires a little work to get it right, but if you do it right you can make it work really well. I run a big High Sierra screwdriver antenna in my Ford Eacape tow hitch from 3 - 30 MHz, and doing all the right things makes it work extremely well on all bands. People think I have a big amplifier when I talk on 75m, 60m or 40m. All I use is my stock 100 watts from my IC-7000, and that does well as long as you have your grounding system all bonded. What actually happens is the SUV couples to the ground below it on the lower bands (you can tell when you detune driving over a bridge or ramp without earth below the vehicle). I especially get a kick out of working anywhere in my state or surrounding states on 40m...all day long. For dawn/dusk operation, I switch to 60m, and then 75m in the dark hours. Incredible how well that all works. But if you are working well now for what you need to do, as is, call it a day and enjoy what you have. Radio is fun stuff, isn't it?

Phil

 

[prcguy]

A tow hitch is still a horrible place to attach an HF antenna even though you bonded lots of stuff. If its working good now you would be surprised how much better it will work if the antenna was mounted up higher on the vehicle. You can probably improve your signal by an S unit on 80m and a half S unit on 40m if there was a higher place to mount it and more centered on the vehicle. A big High Sierra screwdriver is a huge improvement over the smaller ATAS types but there is still room for improvement.

If the antenna system is tuned, grounded and happy then no amount of multiple wavelengths of coax should have an effect on the match. If it does then you probably have common mode currents on the outside of the coax due to inadequate counterpoise, etc.

Another problem with screwdriver antennas is inadequate decoupling of the motor leads and those can radiate and bring RF back towards the radio. The stock choke supplied from most screwdriver mfrs is barely adequate and there are some much better methods to decouple the motor leads on the Internet.
prcguy

Inadequate ground plane (counter poise) would require a longer whip. It's basically the off-center fed concept. Installing braided ground straps to all metal parts on the body and exhaust system (especially your mounting base) will give you a better counter poise and lower your engine noise. Also, some lengths of coax can totally detune a low band antenna. Consider cutting your coax length to odd multiples of the desired frequency wavelength (1/4, 3/4...) by (times) the volicity factor of the cable. This will ensure no nulls within the coax. Even multiples can cancel out the desired frequency. I have see it many times, and you will get maximum bandwidth from the center frequency using odd multiples.

Low band(s) in a mobile installation requires a little work to get it right, but if you do it right you can make it work really well. I run a big High Sierra screwdriver antenna in my Ford Eacape tow hitch from 3 - 30 MHz, and doing all the right things makes it work extremely well on all bands. People think I have a big amplifier when I talk on 75m, 60m or 40m. All I use is my stock 100 watts from my IC-7000, and that does well as long as you have your grounding system all bonded. What actually happens is the SUV couples to the ground below it on the lower bands (you can tell when you detune driving over a bridge or ramp without earth below the vehicle). I especially get a kick out of working anywhere in my state or surrounding states on 40m...all day long. For dawn/dusk operation, I switch to 60m, and then 75m in the dark hours. Incredible how well that all works. But if you are working well now for what you need to do, as is, call it a day and enjoy what you have. Radio is fun stuff, isn't it?

Phil

 

[SOFA_KING]

A tow hitch is still a horrible place to attach an HF antenna even though you bonded lots of stuff. If its working good now you would be surprised how much better it will work if the antenna was mounted up higher on the vehicle. You can probably improve your signal by an S unit on 80m and a half S unit on 40m if there was a higher place to mount it and more centered on the vehicle. A big High Sierra screwdriver is a huge improvement over the smaller ATAS types but there is still room for improvement.

If the antenna system is tuned, grounded and happy then no amount of multiple wavelengths of coax should have an effect on the match. If it does then you probably have common mode currents on the outside of the coax due to inadequate counterpoise, etc.

Another problem with screwdriver antennas is inadequate decoupling of the motor leads and those can radiate and bring RF back towards the radio. The stock choke supplied from most screwdriver mfrs is barely adequate and there are some much better methods to decouple the motor leads on the Internet.
prcguy

Negotive, negootive and negoootive!

I have tried the same antenna with equal bonding on the top of a steel cap (bonded in all four corners). That was the worst performance area of any. Ok on the higher bands, but crap on the lower bands

Tried the back corner, and not great. Had dead spots. Probably too close to the body. Looked nice there, but who cares how it looks if it doesn't work?

I have worked with a friend who put the exact same antenna on his tool box in the bed of his pickup, and he never got anywhere near the signal reports I got. On 40m he had trouble working the stations I could easily work (made him angry). He then moved it to the side of the pickup. Better in some directions, but bad in others. Steak holder mount? Another friend tried that. No bueno, dude.

Low to the ground works incredibly well. That is where that antenna likes to be mounted. The mount spaces the antenna away from the vehicle so it doesn't interfere with antenna, and omnidirectional performance is good in all directions. I have tried it in both my SUV and a work pickup truck. Great results! No one I have heard has gotten better results mounting higher. That is when desperate operators resort to amplifiers, and it still doesn't work.

And you are not correct about coax length. I have fixed many installs by changing the length...did it commercially for a living, so it was my livelihood to fix what no one else could understand.

I have NO problem with RF on my control leads, and NO RF feedback. I know what I'm doing. I added extra chokes...and I will note where you put them can screw up the antenna tuning. Close to the antenna works best.

Sorry. Been there...done that.

Phil

 

[prcguy]

I'll start with a quote from the couch guy, "Low to the ground works incredibly well. That is where that antenna likes to be mounted. The mount spaces the antenna away from the vehicle so it doesn't interfere with antenna".

And I'll respond with no it doesn't work incredibly well and its not where the antenna likes to be. Spacing an HF antenna away from the vehicle body raises the radiation resistance and lowers the efficiency. Lower to the ground continues in the same direction by increasing ground loss and lowering efficiency. Why do you think the OP started this thread? Did he take advise from you???

Avoiding trailer hitch mounts, spare tire brackets and other locations that place an HF antenna off the side of the vehicle body mass are a well documented subject and contrary to your advise. I've done lots of research and testing, which agrees with the industry's findings, but don't take my word for it. Here are some quotes from a well known HF guy on antenna mounting and avoiding the trailer hitch and low spots on a vehicle.

From the K0BG.com site, A website for mobile amateur operators:

"One very important thing to remember about trailer hitch mounting; in terms of efficiency it is the least desirable location. And contrary to popular opinion, running a ground strap to the frame will not negate this fact. If you doubt this premise, read my Antenna Efficiency article. Further, due to the extra ground losses involved, you have to use extraordinary decoupling (large impedance chokes) to keep the RF off of the control and coax leads"

And another quote from K0BG:

"Ground losses (Rg) dominate the efficiency equation, and keeping them low is a worthy exercise. Toward that goal, the most important consideration is proper Antenna Mounting. In other words, It is the metal mass directly under the antenna, not what's along side, that counts!"

I'm not saying K0BG is the end all authority on HF mobile installs, but he is right on track and all my testing agrees with his findings. I never suggested a stake holder mount, those can introduce lots of problems from lack of sheet metal around the base of the antenna.

If you have "fixed" any installs by using a specific length of coax then you didn't fix the actual problem, you just put a band aid on it while the wound is still festering underneath. When the antenna is matched and decoupled properly, no magic length of coax should change the match, it should be the same as a 50 ohm resistive load at the end of the coax. If not, then the coax is part of the radiating antenna and, well, just read the K0BG site and it will save a lot of typing.

Another quote from the couch guy: "I did it commercially for a living, so it was my livelihood to fix what no one else could understand. "

I usually get called last to mop up where people (possibly like you) have put too many band aids on an RF problem and I often have to take things completely back to the beginning of the install and start over or re-enginner the project from scratch. I'm retired now but still get called ocasionally to fly around the world to solve RF, antenna and other communications problems because of my track record in the industry. Question for couch guy, is your phone ringing off the hook for you to fly anywhere an fix anything?
prcguy

Negotive, negootive and negoootive!

I have tried the same antenna with equal bonding on the top of a steel cap (bonded in all four corners). That was the worst performance area of any. Ok on the higher bands, but crap on the lower bands

Tried the back corner, and not great. Had dead spots. Probably too close to the body. Looked nice there, but who cares how it looks if it doesn't work?

I have worked with a friend who put the exact same antenna on his tool box in the bed of his pickup, and he never got anywhere near the signal reports I got. On 40m he had trouble working the stations I could easily work (made him angry). He then moved it to the side of the pickup. Better in some directions, but bad in others. Steak holder mount? Another friend tried that. No bueno, dude.

Low to the ground works incredibly well. That is where that antenna likes to be mounted. The mount spaces the antenna away from the vehicle so it doesn't interfere with antenna, and omnidirectional performance is good in all directions. I have tried it in both my SUV and a work pickup truck. Great results! No one I have heard has gotten better results mounting higher. That is when desperate operators resort to amplifiers, and it still doesn't work.

And you are not correct about coax length. I have fixed many installs by changing the length...did it commercially for a living, so it was my livelihood to fix what no one else could understand.

I have NO problem with RF on my control leads, and NO RF feedback. I know what I'm doing. I added extra chokes...and I will note where you put them can screw up the antenna tuning. Close to the antenna works best.

Sorry. Been there...done that.

Phil

 

[SOFA_KING]

I'll start with a quote from the couch guy, "Low to the ground works incredibly well. That is where that antenna likes to be mounted. The mount spaces the antenna away from the vehicle so it doesn't interfere with antenna".

And I'll respond with no it doesn't work incredibly well and its not where the antenna likes to be. Spacing an HF antenna away from the vehicle body raises the radiation resistance and lowers the efficiency. Lower to the ground continues in the same direction by increasing ground loss and lowering efficiency. Why do you think the OP started this thread? Did he take advise from you???

Avoiding trailer hitch mounts, spare tire brackets and other locations that place an HF antenna off the side of the vehicle body mass are a well documented subject and contrary to your advise. I've done lots of research and testing, which agrees with the industry's findings, but don't take my word for it. Here are some quotes from a well known HF guy on antenna mounting and avoiding the trailer hitch and low spots on a vehicle.

From the K0BG.com site, A website for mobile amateur operators:

"One very important thing to remember about trailer hitch mounting; in terms of efficiency it is the least desirable location. And contrary to popular opinion, running a ground strap to the frame will not negate this fact. If you doubt this premise, read my Antenna Efficiency article. Further, due to the extra ground losses involved, you have to use extraordinary decoupling (large impedance chokes) to keep the RF off of the control and coax leads"

And another quote from K0BG:

"Ground losses (Rg) dominate the efficiency equation, and keeping them low is a worthy exercise. Toward that goal, the most important consideration is proper Antenna Mounting. In other words, It is the metal mass directly under the antenna, not what's along side, that counts!"

I'm not saying K0BG is the end all authority on HF mobile installs, but he is right on track and all my testing agrees with his findings. I never suggested a stake holder mount, those can introduce lots of problems from lack of sheet metal around the base of the antenna.

If you have "fixed" any installs by using a specific length of coax then you didn't fix the actual problem, you just put a band aid on it while the wound is still festering underneath. When the antenna is matched and decoupled properly, no magic length of coax should change the match, it should be the same as a 50 ohm resistive load at the end of the coax. If not, then the coax is part of the radiating antenna and, well, just read the K0BG site and it will save a lot of typing.

Another quote from the couch guy: "I did it commercially for a living, so it was my livelihood to fix what no one else could understand. "

I usually get called last to mop up where people (possibly like you) have put too many band aids on an RF problem and I often have to take things completely back to the beginning of the install and start over or re-enginner the project from scratch. I'm retired now but still get called ocasionally to fly around the world to solve RF, antenna and other communications problems because of my track record in the industry. Question for couch guy, is your phone ringing off the hook for you to fly anywhere an fix anything?
prcguy

What a big bunch of CRAP! You're a fraud. The more you write, the more defensive you appear, and that shows insecurity.

What, did your ego get bruised?

Bottom line is I know what works because it does work, period. I've tried it in many different places, and close to the ground is what gets overwhelmingly good signal reports...sometimes to the point people think I'm running high power or operating at a stationary location with big wire antennas. The SUV body couples to the ground below the vehicle, and that is my ground plane. That is why it works best close to the ground. Simple!

All your hot air isn't going to make one bit of difference, so I'm done with you. Oh, and yes, I actually do get called to fly into situations to solve tough problems and tough RF issues on a critical network that is "life or death" if it fails. That is my job, and I'm very good at it. I get called for the stuff no one else can figure out, so don't even begin to assume you know anything about me...chump.

 

[prcguy]

So you disagree with the info on the K0BG site and pretty much all the other well known antenna guys sites? Does that mean RF flows one way for you and another way for the laws of physics and the rest of us? Do you want the OP of this thread to mount his antenna lower on the trailer hitch to supposedly make it work better?

You might read your post again, the part about "it was my livelihood to fix what no one else could understand". So your stating nobody else knows what you know (even though we know better) and you call me insecure?

I really don't care what you think of me but I do care about suggestions given to others on RR and some of yours is not in line with the way things actually work.
prcguy

What a big bunch of CRAP! You're a fraud. The more you write, the more defensive you appear, and that shows insecurity.

What, did your ego get bruised?

Bottom line is I know what works because it does work, period. I've tried it in many different places, and close to the ground is what gets overwhelmingly good signal reports...sometimes to the point people think I'm running high power or operating at a stationary location with big wire antennas. The SUV body couples to the ground below the vehicle, and that is my ground plane. That is why it works best close to the ground. Simple!

All your hot air isn't going to make one bit of difference, so I'm done with you. Oh, and yes, I actually do get called to fly into situations to solve tough problems and tough RF issues on a critical network that is "life or death" if it fails. That is my job, and I'm very good at it. I get called for the stuff no one else can figure out, so don't even begin to assume you know anything about me...chump.

 

[mikewazowski]

Perhaps we can focus on trying to help the OP rather than hijacking his thread?

 

[prcguy]

Perhaps. I wonder if the OP tried the capacitor trick or had any other success yet?
prcguy

Perhaps we can focus on trying to help the OP rather than hijacking his thread?

 

[wa1nic]

The antenna whip, the base, the rest of the vehicle, are all part of the "antenna".

An ideal 108" whip has an ideal 108" whip below it, pointing in the other direction (some call that a dipole I guess).

If what is below it doesn't look like the other half of a dipole, messing with the upper part to compensate for it makes sense.

I guess it can be nice to fully understand the whole thing, but if it looks like 40-60 ohms resistive at the feed point and nothing is sorted or open... it's Miller time at my house.

Also, fwiw, SWR is only part of the picture. Low SWR certainly helps, but...

What is really important however... what it's all about... is maximum field strength. Everything else just helps to achieve that. If you have access to a field strength meter, put it on top of a metal trash can about 25 feet away and evaluate changes in the antenna by looking at changes in the field strength. That helps one not get fooled by lowering the SWR on a totally hosed installation (like, adjusting to compensate for an unnoticed bad connection).