Cobra 18 WX ST II (don’t laugh)

[ForestRunner98]

I finally replaced a last section on cheap RG-58 A/Unwith my JEFA 240. I know it’s not Times Microwave but it works great for my application.

Ok this is where stuff went weird. In the driveway SWR was a 1.1 BUT when driving it would shoot up to 1.7 to 2.0 on the fwy...WTF!!! So I started the troubleshooting.

Long story short, the right angle connector from the radio was a little loose. I had to remove it when I pulled the radio from the dash. I thought I had fully tightened it. So when driving it would move enough not to make full contact. Truck and road vibrations. It’s cheap and I need to replace with a better one.

I fully tightened and attached my SWR/Watt meter and went for a drive. On the streets SWR hardly moved. On the FWY it barely moved to 1.2 Problem fixed!! I knew something wasn’t right...I put the rig together. So bottom line is always check the simplest most obvious!!

 

[Slowmover]

KNIPEX PLIERS. A range of types but the 5” and 7” as shown are great for the radio toolbox.

Can’t tell you how many times I’ve been in a hurry and forgotten to go back across terminations with one of these. Big truck’em shake loose pronto.

 

[ForestRunner98]

View attachment 45659

KNIPEX PLIERS. A range of types but the 5” and 7” as shown are great for the radio toolbox.

Can’t tell you how many times I’ve been in a hurry and forgotten to go back across terminations with one of these. Big truck’em shake loose pronto.

I like those!! They look safe for connectors. I didn’t want to use regular channel locks because I didn’t want to distort the connector. I tend to King Kong stuff sometimes. LOL

 

[ForestRunner98]

So I decided to get a Nano VNR. Was always curious on how the antenna appeared.

A quick calibrate and it shows that the low spot for SWR is around Ch4. I guess the dip is what people refer it to. My question would be do how to I center that “dip” towards ch20? Shorter or longer antenna? SWR is higher on Ch1 than ch40 so I would think raise the antenna. Any thoughts fellas??

 

[ForestRunner98]

Oh I also ordered a 79” Sirio whip replacement. My current whip is 71” and I can’t get any more height adjustment out of it. So I don’t mind using the taller whip and cutting it down to where it needs to be.

 

[Slowmover]

The below may not matter, but I’ll ask it anyway:

Antenna feedpoint isolation? Shunt coil?

I wouldn’t measure & cut until I’d decided the “antenna mount” is done (used in an all-inclusive sense).

My temp t680 install doesn’t feature all the details. So, antenna tuning (being last, IMO) waits till any incipient OCD has abated once that new truck install is done. (RF Bonds can change whip length, etc).

Granted that whip cutting ain’t the end of the world, but just in case . . . .

.

 

[Handy Andy]

Think in terms of Channels;

While Lower = Longer Length

While Higher = Shorter Length

Then Where is it Resonate - or lowest SWR?

SWR better on Lower channels? Low - means TOO LONG - Make it Shorter.

SWR better on Higher channels? Higher - means too short, make it LONGER.

You can't change the band, but you can change the LENGTH of the antenna to Lower the SWR by finding the "best dip (Resonate or not just low SWR) if it's not on your standby

- adjust the whip for best results on your standby
- most properly mounted antennas will provide a working (not perfect) SWR for the band so tune the dip closest to the side of 20 your standby is on - lower than 20 - longer wavelength - higher than 20 - shorter wavelength.

So - here's an example...

You use 27 as your standby, ok, that's higher than 20.

Where is your Lowest SWR at?

Channel 12? 1:1.2 or so?

Ok, you know that to make the antenna's SWR lower - your BEST SWR is below 20 - so that tells you it is a LONGER antenna than it could be... Make it a little Shorter.

Say, your SWR is lowest on Channel 36?

Ok, Channel 36 is ABOVE 20 - so that means make it LONGER - PULL IT OUT add Length - so lengthen the antenna - longing to be closer to 20.

Remember - Standby is on 27 - so you don't need to change it that much...

The above is thought of as, Frequency is known. You can only adjust the antenna for best SWR - it gives you a Slope Of Direction to take to get the match better.

Your Standby is 27.

Your SWR is lower on Channel 1 than on Channel 40 - Your Antenna is too long - shorten it to raise (slide up) the "dip" to your frequency.

Your SWR is Lower on Channel 40 than on Channel 1 - Your antenna is Too Short - add length to lower the dip (Slide down) to your frequency.​

 

[ForestRunner98]

Good advice guys!! That was a lot!!

I put on the new whip and it was resonate at around high 25 MHz. So it was too long. Also SWR was lower on 1 than 40. Makes sense since the 71” was more plug and play but not where I wanted it.

Pulled out the nippers and started chopping. About a centimeter at first and then smaller as I got closer to 27.205 ch20. Cut a good bit but now the whip sits flush in the coil with it resonate at Ch 20!! 20 now in the “dip” SWR is 1.177 across the 11m band give or take.

That Nano VNA is awesome!! I still want to play and check out what else it does but I am real happy with it. I am sure it’s more accurate than my Radio Shack meter.

I did add 4 per each side of 1” flat braid straps from body to frame. That also made a difference in noise, SWR and getting out!

 

[Slowmover]

Good advice guys!! That was a lot!!

I put on the new whip and it was resonate at around high 25 MHz. So it was too long. Also SWR was lower on 1 than 40. Makes sense since the 71” was more plug and play but not where I wanted it.

Pulled out the nippers and started chopping. About a centimeter at first and then smaller as I got closer to 27.205 ch20. Cut a good bit but now the whip sits flush in the coil with it resonate at Ch 20!! 20 now in the “dip” SWR is 1.177 across the 11m band give or take.

That Nano VNA is awesome!! I still want to play and check out what else it does but I am real happy with it. I am sure it’s more accurate than my Radio Shack meter.

I did add 4 per each side of 1” flat braid straps from body to frame. That also made a difference in noise, SWR and getting out!

H-A’s advice and your description make a nice testimonial to right tools & procedure giving best results.

As to RF Bond, it’s noted several places that horizontal surfaces take precedence (hood, trunk lid, etc). And that exhaust system is an antenna (see DXE kit).

.

 

[ForestRunner98]

H-A’s advice and your description make a nice testimonial to right tools & procedure giving best results.

As to RF Bond, it’s noted several places that horizontal surfaces take precedence (hood, trunk lid, etc). And that exhaust system is an antenna (see DXE kit).
.

That’s so true!! A Rig Expert analyzer would have been nice but the price was out of my range. If I did more installs I could see the benefit. But for $55 that Nano performed well. I watched some YouTube channels on how to calibrate it. Not difficult at all...at least checking for SWR.


Already been done. Just those 8 straps from body to frame was the last piece. Used eyelets, crimped and soldered.

 

[Slowmover]

So, the antenna system is “done”, is that right?

Power: A half-volt drop and no more?

What’s left is:

1). Noise Abatement? DSP + maybe some filters or ferrites?

2). Field Strength Chart?

3). Some mobile-to-mobile distance testing?

Don’t mean today or tomorrow, but the next steps where pride has its place.

After all, you pulled out and passed this old slow mover a while back. I’m wondering what the road looks like.

.

 

[ForestRunner98]

So, the antenna system is “done”, is that right?

Power: A half-volt drop and no more?

What’s left is:

1). Noise Abatement? DSP + maybe some filters or ferrites?

2). Field Strength Chart?

3). Some mobile-to-mobile distance testing?

Don’t mean today or tomorrow, but the next steps where pride has its place.

After all, you pulled out and passed this old slow mover a while back. I’m wondering what the road looks like.

.

Ya it’s basically done at this point. But you know how that goes...LOL

As for voltage drop I haven’t checked on that but the alternator is a few months old and the 203 really powers up when the truck is on.

Just looking at DSP but as of now it’s not my top priority. I just don’t use it enough to really benefit but I can see the advantage of it. There are are ferrites at the feed point, a toroid choke and a Palomar CMF-500 at the radio. There is slight ignition “pop” but it’s coming into the antenna. Coax disconnected and there is absolute silence. Trying to fix that is a challenge. But the McKinley NB/ANL does a great job of silencing it!

I do have good mobile to mobile contact. 35 miles or so non skip conditions on AM. So far 60 miles is the best I have done on 19am. Not too bad from a 203.

Pass SLO up? Na!! They’re just two different type vehicles that have their own challenges. Future?? Looking at a small 2 pill or maybe 4. I can use the 203 to drive it. Depends on what Mr. Wallet says!!

 

[ForestRunner98]

I wanted to see where that resonate “dip” was when driving. It falls perfect while static but moving...let’s see.

Hooked up the VNA and went for a small drive. Backing up a few feet from house made a shift as expected. BUT....when I cleared the house the “dip” went back to where I tuned it. So let’s drive. “Dip” hardly moves when moving!! Oh ya!!! Would only shift at a stop light that had the big metal pole and light over the car. Antenna shows long. Green light go...back to normal.

Definitely happy with this VNA and new longer whip! Oh I also had a chit chat with some locals and got some great feedback!!

Sorry I am just excited and I wanted to report some real world numbers.

 

[Shadetree Mechanic]

I wanted to see where that resonate “dip” was when driving. It falls perfect while static but moving...let’s see.

Hooked up the VNA and went for a small drive. Backing up a few feet from house made a shift as expected. BUT....when I cleared the house the “dip” went back to where I tuned it. So let’s drive. “Dip” hardly moves when moving!! Oh ya!!! Would only shift at a stop light that had the big metal pole and light over the car. Antenna shows long. Green light go...back to normal.

Definitely happy with this VNA and new longer whip! Oh I also had a chit chat with some locals and got some great feedback!!

Sorry I am just excited and I wanted to report some real world numbers.

That's awesome! I never thought about doing that! I'm going to hook up my nano and see what it does in the different places I park. Thanks for the idea.

 

[ForestRunner98]

That's awesome! I never thought about doing that! I'm going to hook up my nano and see what it does in the different places I park. Thanks for the idea.

Ya man try it! Being mobile I figuered it’s good to know what your antenna does when moving or parked in other areas. Besides my OCD was kicking in. LOL

Posting my testing/results is my small(lamen) way of giving back to this awesome site!