View Full Version : Does Coax Type and Length REALLY Make a Difference in a Mobile Set-Up?

12-28-2021, 03:24 PM
"Hello" Everyone,

I have a question, that I need a few of you experts out there to comment on, if you will.

Mobile Set-up:

Midland 5001Z- Radio
Astatic D104-M6B- Microphone
Wilson 1000 Magnetic Mount w/ RG58 A/U Coaxial Cable (18Ft)- Antenna

I have a 2nd Magnetic Mount- 3/8X24- w/RG58 A/U Coaxial Cable (12Ft).

I intend to use this, for when skip conditions are advantageous. So, far though I have only used for local talk...and have only used each antenna once...just to see if they worked. They both work really well but have not tuned them yet. I don't want to use them a lot until I can check the SWR.

I have two antennas I plan to use with this mount:

-7Ft. Skip Shooter Fiberglass antenna
-President Texas Steel Whip antenna (Just shy of 7Ft.)

This would be my... park and talk, mount and antenna system.

With my Wilson 1000. I can drive about 1 mile and be on top of a pretty high elevation, compared to the surrounding area. I talk on a NET on Friday nights... and from this position, I can always talk to a
friend with a base station... 24-25 miles away. And he always, hears me loud and clear.

I have not used my other antennas at this spot yet.

I was thinking of changing the coax, from both mounts to LMR-200.

With the length of coax, I am using. Would, it make ANY difference in the over-all performance of
my mobile set-up? Or would it be a waste of time?

I look forward to hearing your input on this. Thank-you, in advance.

P.S- President recommends using less than 13Ft. of coax for their President Texas antenna.
Can anyone explain to me why that is? That is why, I bought the 2nd Magnetic mount...
because it came with 12Ft. of the above said coax... and now, I can experiment with different antennas because of the 3/8X24 mount. My Wilson 1000 has a different type of connection.

When I use my 2nd Magnetic mount. I remove my Wilson 1000 antenna, but leave the mount
on. And I put the 2nd mount, right in front of my Wilson mount... on top and in the middle of the
cab of my Toyota Tacoma.

The DB
12-29-2021, 12:33 AM
Short answer, no. Making this change in coax won't make a hill of beans worth of difference in this case.

Assuming channel 20 on the CB band, 18 feet of Belden 9201 RG-58 (their lower quality version) will net you 0.358 dB of loss when running it into an SWR match, and 0.439 dB of loss if run into an SWR of 2.

LMR-200, again 18 feet, will improve these numbers to 0.303 dB of loss with a perfect match, and 0.373 into an SWR of 2. There will be a whopping change of 0.055 dB for a perfect match, and a change of 0.066 dB for an SWR of 2.

To put that in perspective, you won't notice a difference with anything less than about 3 dB of change. The numbers above are nothing, they don't even come close to anything you will ever notice.

If you do change out coax and you notice a difference, it will be because you had a bad length of coax to begin with.

I used this coax loss calculator (https://www.qsl.net/co8tw/Coax_Calculator.htm) to get the numbers above.

The DB

12-29-2021, 12:46 AM
Thank-you, DB for taking the time to crunch some numbers and offering a great reply. Thanks also for the link to the coax loss calculator. I had a feeling it probably wasn't worth the time. I have no issues with my radio performing well.
But always looking for a way to improve things. Thanks again for your help.

12-29-2021, 10:33 PM
With coax seldom being a factor, when dealing with full length hardware (102 inch), the physical location has more to do with propagation and SWR which can do more harm than small amounts of attenuation. Remember that each connector adds -.2 db attenuation, keep them to a minimum and solder them on, not crimped.

Cedar Mountain Radio
12-30-2021, 10:21 AM
Hi Vagabond,
I was reading the above posts and one thing I didn't see addressed was RF getting into the equipment by riding the coax back in and causing distortion or squealing in the mic. I found that coax loss isn't really the bigger factor in mobile antennas because the coax length is usually so short that the losses are miniscule, but to me, having a length that doesn't cause problems with RF feedback is the more important issue.
I find that using 17-18 feet of any type of coax often causes RF feedback problems and I attribute that to it being a half wave of shield and that puts a lot of voltage at each end.
I have done away with RF distortion by getting away from a half wave length or about 18 feet and going to more like 12 or 24 feet when I was getting distortion or squealing in the mic.
Maybe that helps.

02-27-2022, 07:31 AM
What a load of of crap, .2 @ 27mhz? Including crimp v solder, that's just not true at all.

Alabama Buckeye
03-10-2022, 02:36 AM
Really? No attenuation? You have the right answer? All I see wrong in that comment is the algebraic negative -.2. That is a subtraction of attenuation; which I see as a simple stray pinky error.

The DB
03-10-2022, 06:18 PM
For some reason, in this hobby, people tend to overestimate very minor differences in things...

A PL-259 to SO-239 connection does not have 0.2 dB of loss. The real figure is closer to 0.025 dB of loss, as measured by my old AIM4170C VNA. I've seen several youtube videos where others have done a similar test and got very close to the same results. This was also confirmed more recently using my Nano-VNA. So in this Archy was correct, 0.2 dB of loss is an overstatement.

I didn't see where anyone said or implied that there was no attenuation, so that line has me confused. The closest I've seen to this is Archy saying that 0.2 dB of loss at 27 MHz wasn't true, which is correct., but this is not saying there is no attenuation.

The difference between a soldered and crimped connector is far less than most people who talk about it will ever realize. Yes, there is a difference, but the difference is so small that it is negligible. If you notice a difference between a soldered and crimped connection, then one of them was not done correctly. In fact, I would say a high quality crimped connector will likely have less attenuation than a low quality soldered connector, This is another example of blowing very minor differences out of proportion.

And finally, when it comes to decibels and notation. Just because something is being shown as a loss in decibels does not mean it is represented as a negative. Return Loss, for example, is a measurement of loss in decibels, but is represented as a positive number, and this is also this way on professional grade equipment. As long as it is made clear that we are referring to a loss in decibels, the negative sign is not necessary.

The DB

03-10-2022, 10:19 PM
Thanks to your comment, I don't have to bother replying to the other comment.
The 0.2 dB thing probably came from a datasheet for a much higher frequency.
As for soldering plugs, there are complications there. People think they've soldered the shield properly, but often it is not. They put a tremendous amount of heat through the plug and coax which can damage the dielectric of the plug potentially making it more lossy, especially under power. The heat can damage the coaxial dielectric changing its properties. Many solder foam dielectric type coaxials like LMR400 which is strictly designed for crimp or compression plugs only. They destroy the dielectric on the end of the coax as it melts quite easily. Remember people, coax conductor spacing and dielectric properties are critical to the specs.
You can argue that solder is lossy. As you say, the crimp and compression mechanical type connections are fine, it's an industry standard. After all, the plugs and sockets are mechanical anyway.
Lastly, people focus so much on having an extra connector or adapter when the coax they are using is of poor quality, old or damaged. Or they could upgrade to a lower loss cable for much greater gains in minimising attenuation. The attenuation of connections at 27mhz is negligible.

Alabama Buckeye
03-11-2022, 06:06 PM
I'm one of those who cant solder worth a nickle. I'm the "Buckeye" for more than I like Alabama Buckeye blossoms, and the nut; but I'm adaptable. I use 8X coax with PL-259 with the handy dandy adapters. All my connections coax to PL-259 since have resulted in 0 ohms resistance. I use a flaring technique on the shield using contact pressure alone and solder only the center electrode. I can't tell you how many cable shields I've brushed and picked apart into straight strands. I get 1.3:1 or less with all my fittings; better than some factory made jumpers. Buckeyes are edible after a lot of work leaching them, like acorns.

Alabama Buckeye
03-11-2022, 06:57 PM
I'm curious about the President, the Texas Steel 7' Whip and it's reported 7/8 wave performance. That's a lot of wound up wire in that bottom load. That's 24.5' of wire after subtracting 7'. Can't quote but have read it on numerous sightings where 5/8 wavelength is the best aside from a 36' dipole. Why President didn't fatten the load space and wrap for a full wavelength is another curiosity. I can imagine "7/8 wave" and "weather trap" as two glaring Advertising Blunders? They trap water in the loaded base to immerse the windings in the corrosion.

As to coax, LMR200 is quite good as it has two shields. And it's not LMR400, which is >twice the diameter. LMR200 has the diameter of the popular RG58 at 3.6x the price. Having said that, I'd take tender care of that LMR200. No crimping in the window or door jamb.

A real experts would know better than I. It has been my practice to have coax length in multiples of 3'. 13' wouldn't fit but 12 or 18 would. Why? Because that's what the experts said. By Definition: Expert - Bygone drip under pressure. I was once an expert now I'm retired.