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SWR trouble
- Thread starter my2995dx
- Start date
[So everyone knows - this is posted in CB Antennas section of the Forum]
There's a lot to be said about mounts.
Ok, so I have this straight - 3 different antennas...
Magnet mount used - but on all 3 antennas?
Fender Mount (ugh!) - on all 3 antennas?
We need a little more information - like where you mounted the antennas on the vehicle - because if the location was good but the mount has issues, or the location and mount were good - but the antennas have issues - sort of thing...a process of elimination. Rules out certain things or stuff done - so we can focus on your mess with - er, whatever is the issue(s)
There's a lot to be said about mounts.
Ok, so I have this straight - 3 different antennas...
Magnet mount used - but on all 3 antennas?
Fender Mount (ugh!) - on all 3 antennas?
We need a little more information - like where you mounted the antennas on the vehicle - because if the location was good but the mount has issues, or the location and mount were good - but the antennas have issues - sort of thing...a process of elimination. Rules out certain things or stuff done - so we can focus on your mess with - er, whatever is the issue(s)
I have to agree with Andy & freecell. Lot of factors involved as I've dealt it here recently with both a class 8 truck and the trunk lid of the sedan. Location, antenna type (top/bottom load), mount type. Bonding seems to be a must in just about most newer cars, I found the to be the biggest culprit next to location.
More thorough details really help
More thorough details really help
For what purpose? We're looking for a RF ground, not a DC ground. The battery negative plays absolutely no role in that. You don't even need to have the antenna mounted on the same vehicle to provide a decent ground, you could have a magmount on the roof of the car parked next to you and it would use that vehicle's roof and car body as the groundplane.
Beg, borrow or buy an antenna analyser to see what's going on - you can get cheap NanoVNA ones for under $50. If you've a very good RF ground you can find that because manufacturers make antenna lengths assuming it'll be installed with a crap RF ground like the majority are, that it's actually resonant below the CB frequencies out of the box as the more efficient the RF ground the shorter the antenna needs to be for a given frequency. If your SWR on your lowest channel is lower than the SWR on your highest channel then this does indicate this is what is happening.
The problem with trying to do it with just a CB if you've the issue above is that even on the lowest channel it doesn't go low enough in frequency to the point where the antennas lowest SWR is.
I'm lucky that I have an analyser and every CB antenna I get is usually resonant around 26MHz out of the box and I have to cut them down. The Sirio 5000 I have I had to cut three inches off. You don't want to be shortening them by cutting them until you're sure that's what the situation is.
Yup!! I had to cut about the same amount of whip off my Sirio 5000. Got a $60 VNA. I wouldn’t have been able to do it with just a SWR meter.
Here is the result. I think it’s pretty good.
@ForestRunner98 Nice Job...
You see where you are "at" on the "Smith Chart" background?
You have an ever so slight "+j" component along with a impedance bump the antenna cannot be used to compensate for - Your SWR "Circle" and loop back shows this.
No matter what though - you're nearly spot on - and with some CB Radios with built in SWR detectors - this would have not even "moved" a needle even with the SWR CAL knob cranked for full sensitivity.
Just fun to watch you work - you're doing great!
You've come a long way - your family should be proud of you
You see where you are "at" on the "Smith Chart" background?
You have an ever so slight "+j" component along with a impedance bump the antenna cannot be used to compensate for - Your SWR "Circle" and loop back shows this.
No matter what though - you're nearly spot on - and with some CB Radios with built in SWR detectors - this would have not even "moved" a needle even with the SWR CAL knob cranked for full sensitivity.
Just fun to watch you work - you're doing great!
You've come a long way - your family should be proud of you
Thank you @Handy Andy!!!
I could not get it any better. I tried but was at the limit of my knowledge on the matter. I saw that it’s at 40ohm and not 50. On a external SWR meter there is no bump in SWR. Obviously the VNA sees the truth. So the VNA is really a super useful tool.
I will say one thing is that this mother trucker radio gets out and has dumbo ears to match!! If I had more fire in the wire who knows? LOL
I appreciate the the kind words Handy!! Started my journey with a 3’ Firestik with a bracket off the tail light and slowly progressed to where I am now.
I can’t say it was all me. I had a TON of help and support from all of you guys!!! I am sure my family is proud but they still look at me weird on this! LOL
PS...if there is a way to make things better please let me know!!
the vna is also providing information about resonance. the 24.4 nH figure is indicating that the antenna is inductively reactive yet close to resonance but a bit too long @ 27.200 mhz.. resonance is somewhere just below that frequency. 24.4 nH = .0244 uH. in the case of a loaded antenna this adjustment will be "touchy" but it would be interesting to slide down below 27.200 mhz. to verify,. this can be confirmed as the 24.4 nH. figure is reduced. swr will drop slightly to 1.2388:1 when the inductance is cancelled at which point the (load) antenna becomes a pure resistance. good work.
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I do have the original Sirio whip which is shorter than the longer Sirio whip I am using. 27.200 is where the dip is the lowest.
I will put back the shorter whip in the morning and see what numbers the VNA sees. Never used the VNA on the shorter whip. Should be interesting.
I just realized that I went longer rather than shorter but the SWR looks better. Still don’t understand the 19.3nH reading. It’s lower than the first pic. Good or bad?
The best way to review this is to see the Smith Chart it overlays onto - which to me, is a big help in determining "Where" the antenna is, and "Where" you want to go.
The VNA is showing you where the Antenna "meets" Lowest SWR - even if you "zoom in" you're still around 1:1.2 with the antenna having a "+j - Component" which shows reactive - and ANYTHING using Open LEAD mounts - read this as simple 3/8 - 24 threaded - is "open lead" and then to have this as the results of the antennas own reactive elements working with and against the signal - your losses are Negligible the Yellow and Blue points are really close together - you have more to lose trying to beat the SWR to the Blue point than it's worth.
Also remember too - that with open lead mounts - the VSWR shows more readily because of inherited Connection and Joint mismatch "bumps" in the impedance than the antenna may cause. IT's no ones fault and nor is it anything truly to worry about.
This is normal - don't worry - you're fine!
To help understand this - you have 2 things against you in mounting - one is location - the other is the losses in the type of mount used - the simpler mounting schemes are the least lossy. So a 3/8-24 threaded types are about the best, but remember the "cut" in the coax line, the Lugs to attach the antenna and the ground counterpoise to this stud and coax - all play a bigger role in the RESISTIVE losses which add to your problem in dissimilar metals and the "spacing" of the mount elements to attach the mount to the location and the antenna to the mount - they are NOT always equal NOR are they the best, there's another "Bump".
So you're in good (great shape really) to continue using the system and not have any true worries except the durability of the mount and the antenna to survive the elements of Mother Nature - it's not all sunny skies and Rainbows out there.
The VNA is showing you where the Antenna "meets" Lowest SWR - even if you "zoom in" you're still around 1:1.2 with the antenna having a "+j - Component" which shows reactive - and ANYTHING using Open LEAD mounts - read this as simple 3/8 - 24 threaded - is "open lead" and then to have this as the results of the antennas own reactive elements working with and against the signal - your losses are Negligible the Yellow and Blue points are really close together - you have more to lose trying to beat the SWR to the Blue point than it's worth.
Also remember too - that with open lead mounts - the VSWR shows more readily because of inherited Connection and Joint mismatch "bumps" in the impedance than the antenna may cause. IT's no ones fault and nor is it anything truly to worry about.
This is normal - don't worry - you're fine!
To help understand this - you have 2 things against you in mounting - one is location - the other is the losses in the type of mount used - the simpler mounting schemes are the least lossy. So a 3/8-24 threaded types are about the best, but remember the "cut" in the coax line, the Lugs to attach the antenna and the ground counterpoise to this stud and coax - all play a bigger role in the RESISTIVE losses which add to your problem in dissimilar metals and the "spacing" of the mount elements to attach the mount to the location and the antenna to the mount - they are NOT always equal NOR are they the best, there's another "Bump".
So you're in good (great shape really) to continue using the system and not have any true worries except the durability of the mount and the antenna to survive the elements of Mother Nature - it's not all sunny skies and Rainbows out there.
