Beyond confused

[338_MtRushmore]

I've been messing with the antenna system on my pickup for a while now. It's been pretty frustrating because it seems like none of my results align with anything I thought I knew. The vna would say one thing, and the meter would say another. I chased my tail for a while adding chokes and a low pass filter, retuning and it just wasn't right.

Today I decided to start over. I did the 1/4 wave coax that cbers love and hams despise. I was planning to tune for x, then try my hand at a matching network. I ended up with this.


The simple fact that it is over 50 ohms confused me as I expected it to be lower than 50. The swr at the amp was actually pretty close to the vna readings, so that was a step in the right direction. For some reason I decided to add an mfj choke along with a 3 foot jumper and the swr at the amp stayed under 1.1:1. I'm thinking I'm just going to run it like this.

This project has proven to me that I know less than nothing about antennas, and what I thought I knew is totally worthless in the real world.

 

[sp5it]

Why 1/4 coax? Is it 75 or 50 Ohm cable?
What is the antenna you are using?

 

[ForestRunner98]

Isn’t a 1/4 wave 9’? The CB coax “length” is 18’ making it 1/2 wave.

 

[The DB]

As stated above, many on the CB side swear by the 1/2 wavelength, although based on your posts on this and other forums in the past I am sure you are aware of that and simply mistyped.

What happens to the tune if you use a barrel connector and add a two or three foot jumper? Optionally you can reroute the coax in the vehicle and see if that changes the results.


The DB

 

[338_MtRushmore]

Well I did say I know less than nothing. I actually used a full wave because I didn't want to to hack up my coax for a failed experiment.

 

[sp5it]

Multiplication of 1/2 wave is OK.

 

[338_MtRushmore]

As stated above, many on the CB side swear by the 1/2 wavelength, although based on your posts on this and other forums in the past I am sure you are aware of that and simply mistyped.

What happens to the tune if you use a barrel connector and add a two or three foot jumper? Optionally you can reroute the coax in the vehicle and see if that changes the results.


The DB

I removed the line isolator and added a barrel connector and it looks like my swr curve moved up about 200Khz

Out in the open my swr with the isolator is 1:1 on 27.500 and 1.1:1 on 27.385. In my driveway I am 1:1 on 27.185 and 1.1:1 on 27.385. Seems like a pretty happy medium for now.

 

[Marconi]

338, do you know why your VNA is showing in blue, "Ls out of range?"

Do you or anyone else know what this means?

Could this be a calibration error?

It looks to me there maybe another error noted in red, "HW cal NO."

 

[338_MtRushmore]

View attachment 60151

338, do you know why your VNA is showing in blue, "Ls out of range?"

Do you or anyone else know what this means?

Could this be a calibration error?

I don't know. This one always says that when x is zero, or at least when it is between -0.2 and 0.2. Basically anytime it reads x=>5k it also reads Ls out of range.

I also don't know what "HW cal" is either. Measurements don't mean squat if they are inaccurate or interpreted incorrectly.

 

[338_MtRushmore]

I did figure out how to do the HW calibration, but I must have flashed new firmware at some point. I'm going to go back through the entire setup and calibration again.

 

[338_MtRushmore]

After a double check of settings and recalibration, it didn't really change much.

Before (yesterday)

After (today)

 

[Captain Kilowatt]

Isn’t a 1/4 wave 9’? The CB coax “length” is 18’ making it 1/2 wave.

And those numbers are physical lengths and have nothing to do with the electrical lengths. You need to factor in the cable's velocity factor to arrive at the fabled magic length.

 

[M0GVZ]

With those VNAs you need to do the calibration every time you change the cable you're running from the VNA to the antenna feedpoint if you want accuracy.

 

[Crawdad]

One must never ignore a perfectly good myth when in pursuit of the Magical, Mystical 1:1 and all of the Mythical goodness it brings.

Here's wishing that RF would propagate as well as the myths do. No F2 layer required.

 

[338_MtRushmore]

With those VNAs you need to do the calibration every time you change the cable you're running from the VNA to the antenna feedpoint if you want accuracy.

Even if I pulled the headliner to access the mount, would I just solder a resistor between the ring terminals? I don't think I can add a resistor at the antenna mount because there is essentially 2" of antenna in the mount itself. This just doesn't seem very feasible.

If I recall correctly my coax measured r=0.9 and x=0. Is that too far from zero to be useful? I thought that was the whole point of the "myth," and why hams even say to measure at the feedpoint or with a half wave coax.