Now I'm a believer

[WRTC928]

I recently installed some NMO mounting points on my truck, but for a variety of reasons, I wasn't entirely satisfied with them and decided to do it over. For the first installation, I used generic Chinese RG8X coax, but this time I went with USA-made MPD digital RG8X. The first thing I noticed was that the MPD coax is much more flexible and easier to work with, but even more significant is what happened to my SWRs/Smith graphs. With the Chinese coax feeding a Diamond SG7900 Super Gainer NGP 2m/70cm dual-band antenna, the 2 meter and 70 centimeter bands were good, GMRS was in the "I can tolerate it if I don't use it too often" range and MURS (Which I've never used and could live without, but why not test it?) Started at 2.0:1 and went up from there. The first four pics are the sweeps of GMRS, 2m, 70cm, and MURS in that order with the Chinese coax and the last four are the same after changing to the MPD coax on the same antenna. 2m and 70cm remained good and even improved a little, but GMRS and MURS improved dramatically -- all the way to, "Yeah, that'll do." If (like me) you have sometimes wondered, "Does it really make that much difference?" the answer is yes, it does. The MPD ain't cheap at $2.34/foot, but I only needed 30 feet. TBH, even if I needed 100 ft, I'd buy the MPD. The difference is that obvious. As the saying goes, if you buy the best, you only cry once.

 

 

[marcspaz]

That's really something. Even with such a short length, it makes a big difference. 

[WRTC928]

32 minutes ago, marcspaz said:

That's really something. Even with such a short length, it makes a big difference. 

I was surprised. On the forums, there's always someone saying how Chinese coax is crap, real men only by USA cable, etc., but nobody's ever shown me the actual results. This is only one specific set of circumstances, and it's possible that if I had a good ground plane under the antenna, or a different antenna, the difference wouldn't be so obvious. Maybe in some cases, there's not enough difference to make a difference (RIP Paul Harrell), but in this case, it was worth the extra expense.

[SteveShannon]

That does make a difference, but what is the actual difference?  How much of the difference is due to the antenna?  Are the cables exactly the same length? Have you tested the insertion loss for each cable at each tested frequency?  Remember, attenuation in an antenna makes SWR appear better.  

I believe these are the tabular results and I’ve arranged them by frequency  It appears the older cable actually results in lower SWR for the 2 meter and 70 cm bands when connected to an antenna designed for those frequencies according to the value in the upper left of the screen. It would be interesting to see a single wideband sweep of just the cables into a dummy load to eliminate the effects of the antenna.

Frequency     Old Cable SWR  ->   New Cable SWR 

462.000-470.000. SWR 1.900:1 -> 1.495:1

441.000-450.000.  SWR 1.464:1 -> 1.498:1

151.800-154.600.   SWR 1.888:1  -> 1.408:1

144.500-146.400.  SWR 1.151:1 -> 1.395:1 

[Mrsig]

Thanks for posting.

[H8SPVMT]

Some fancy pictures you have there! How did you draw them maybe a swirl o'graph?   I had a SWR meter on the CB but it was a needle and flat graft of numbers.

Reminds me of the ole navagation system Loran C location mapping.....

 

[WRQI663]

He's using a nanovna -- it has the smith chart on it.

[WRTC928]

2 hours ago, SteveShannon said:

That does make a difference, but what is the actual difference?  How much of the difference is due to the antenna?  Are the cables exactly the same length? Have you tested the insertion loss for each cable at each tested frequency?  Remember, attenuation in an antenna makes SWR appear better.  

I believe these are the tabular results and I’ve arranged them by frequency  It appears the older cable actually results in lower SWR for the 2 meter and 70 cm bands when connected to an antenna designed for those frequencies according to the value in the upper left of the screen. It would be interesting to see a single wideband sweep of just the cables into a dummy load to eliminate the effects of the antenna.

Frequency     Old Cable SWR  ->   New Cable SWR 

462.000-470.000. SWR 1.900:1 -> 1.495:1

441.000-450.000.  SWR 1.464:1 -> 1.498:1

151.800-154.600.   SWR 1.888:1  -> 1.408:1

144.500-146.400.  SWR 1.151:1 -> 1.395:1 

It is certainly interesting when you chart it that way. As you say, the SWR improved on the bands for which the antenna was not designed, but not on the bands for which it was designed. In effect, it made this antenna a little more broad-banded. Now I need to do some more testing. I'll try a couple of different antennas and maybe the dummy load test. 

[Socalgmrs]

8 hours ago, WRTC928 said:

it's possible that if I had a good ground plane under the antenna,

So you have a ground plane antenna with out a proper ground plane?  Why bother you neutered your antenna.  The whole system is not good at that point.  You probably 1/4erd your tx distance.  All that time and money wrapped up in a bad ground plane makes the rest pointless.  

[tcp2525]

RG8x on UHF? Definitely not a optimal choice no matter where it's made. I use RG400 on all my NMO mobile installs. It's quality stuff even if it's made in an Asian country. 

[SteveShannon]

11 hours ago, WRTC928 said:

I was surprised. On the forums, there's always someone saying how Chinese coax is crap, real men only by USA cable, etc., but nobody's ever shown me the actual results. This is only one specific set of circumstances, and it's possible that if I had a good ground plane under the antenna, or a different antenna, the difference wouldn't be so obvious. Maybe in some cases, there's not enough difference to make a difference (RIP Paul Harrell), but in this case, it was worth the extra expense.

Those SWR numbers are not accurate.
Keep in mind that SWR is calculated by the following formula:

SWR = (Forward Power + Reflected Power) / (Forward Power - Reflected Power) 

But that requires an accurate measurement of the percentage of the reflected power compared to the original power output.  At GMRS frequencies RG-8x converts 44% of the RF output to useless heat before it ever reaches the antenna. So of the maximum of 50 watts transmitted by a GMRS radio only 27 watts reaches the antenna. That’s the Forward Power. 
Then, if the impedance of the antenna isn’t perfect, some amount of the power is reflected back towards the radio. That’s the Reflected Power. By the time it reaches the radio 44% of the reflected power has been converted to heat.

So, let’s say you have an SWR meter right at the radio and let’s also assume that the antenna truly has an SWR of 10.0:2 at 462 MHz. An SWR of 10.0:1 means 67% of the forward power is reflected. That’s a pretty abysmal SWR for any antenna. So for a 50 watt transmitter 33 watts reflects off the feed point of the antenna and back towards the radio. 
But now let’s introduce attenuation. 50 watts doesn’t really reach the antenna. We’re using RG-8x so only 27 watts reaches the antenna. Two-thirds reflects  back towards the radio, so only 9 watts of power is radiated instantly. 18 watts is the reflected power at the feed point but of course 44% of that becomes heat due to attenuation. 56% of 18 watts, or 10.1 watts makes it back to the SWR meter. So the SWR meter sees all 50 watts forward power and only 10.1 watts reflected power. It’s calculation says that is a perceived SWR of about 2.6:1.

The point is that lossy coax results incorrect SWR measurements, whether done by a nanoVNA, an expensive bench model Vector Network Analyzer, or the priciest antenna analyzer out there and seeing a lower SWR becomes meaningless.  
 

[tcp2525]

1 hour ago, SteveShannon said:

Those SWR numbers are not accurate.
Keep in mind that SWR is calculated by the following formula:

SWR = (Forward Power + Reflected Power) / (Forward Power - Reflected Power) 

But that requires an accurate measurement of the percentage of the reflected power compared to the original power output.  At GMRS frequencies RG-8x converts 44% of the RF output to useless heat before it ever reaches the antenna. So of the maximum of 50 watts transmitted by a GMRS radio only 27 watts reaches the antenna. That’s the Forward Power. 
Then, if the impedance of the antenna isn’t perfect, some amount of the power is reflected back towards the radio. That’s the Reflected Power. By the time it reaches the radio 44% of the reflected power has been converted to heat.

So, let’s say you have an SWR meter right at the radio and let’s also assume that the antenna truly has an SWR of 10.0:2 at 462 MHz. An SWR of 10.0:1 means 67% of the forward power is reflected. That’s a pretty abysmal SWR for any antenna. So for a 50 watt transmitter 33 watts reflects off the feed point of the antenna and back towards the radio. 
But now let’s introduce attenuation. 50 watts doesn’t really reach the antenna. We’re using RG-8x so only 27 watts reaches the antenna. Two-thirds reflects  back towards the radio, so only 9 watts of power is radiated instantly. 18 watts is the reflected power at the feed point but of course 44% of that becomes heat due to attenuation. 56% of 18 watts, or 10.1 watts makes it back to the SWR meter. So the SWR meter sees all 50 watts forward power and only 10.1 watts reflected power. It’s calculation says that is a perceived SWR of about 2.6:1.

The point is that lossy coax results incorrect SWR measurements, whether done by a nanoVNA, an expensive bench model Vector Network Analyzer, or the priciest antenna analyzer out there and seeing a lower SWR becomes meaningless.  
 

Yep, he built a dummy load with that cable. Silly rabbit. 

[WRTC928]

15 minutes ago, tcp2525 said:

Yep, he built a dummy load with that cable. Silly rabbit. 

Alright, smart guy. How do you hook up an antenna without using a cable?

[UncleYoda]

Maybe 2 tests, one at the radio and one with meter at antenna end of cable.  Compare the two results.

[WRTC928]

1 hour ago, SteveShannon said:

Those SWR numbers are not accurate.
Keep in mind that SWR is calculated by the following formula:

SWR = (Forward Power + Reflected Power) / (Forward Power - Reflected Power) 

But that requires an accurate measurement of the percentage of the reflected power compared to the original power output.  At GMRS frequencies RG-8x converts 44% of the RF output to useless heat before it ever reaches the antenna. So of the maximum of 50 watts transmitted by a GMRS radio only 27 watts reaches the antenna. That’s the Forward Power. 
Then, if the impedance of the antenna isn’t perfect, some amount of the power is reflected back towards the radio. That’s the Reflected Power. By the time it reaches the radio 44% of the reflected power has been converted to heat.

So, let’s say you have an SWR meter right at the radio and let’s also assume that the antenna truly has an SWR of 10.0:2 at 462 MHz. An SWR of 10.0:1 means 67% of the forward power is reflected. That’s a pretty abysmal SWR for any antenna. So for a 50 watt transmitter 33 watts reflects off the feed point of the antenna and back towards the radio. 
But now let’s introduce attenuation. 50 watts doesn’t really reach the antenna. We’re using RG-8x so only 27 watts reaches the antenna. Two-thirds reflects  back towards the radio, so only 9 watts of power is radiated instantly. 18 watts is the reflected power at the feed point but of course 44% of that becomes heat due to attenuation. 56% of 18 watts, or 10.1 watts makes it back to the SWR meter. So the SWR meter sees all 50 watts forward power and only 10.1 watts reflected power. It’s calculation says that is a perceived SWR of about 2.6:1.

The point is that lossy coax results incorrect SWR measurements, whether done by a nanoVNA, an expensive bench model Vector Network Analyzer, or the priciest antenna analyzer out there and seeing a lower SWR becomes meaningless.  
 

It sounds like you're saying there's no point in testing. Any cable that's practical for a vehicle application will be lossy. RG58 is even worse, and that's what pretty much all mag mounts and NMO kits include. Are you suggesting we should use LMR400 for vehicle antennas? Why do you assume the antenna has an SWR of 10:1? 

[LeoG]

2 hours ago, SteveShannon said:

Those SWR numbers are not accurate.
Keep in mind that SWR is calculated by the following formula:

SWR = (Forward Power + Reflected Power) / (Forward Power - Reflected Power) 

But that requires an accurate measurement of the percentage of the reflected power compared to the original power output.  At GMRS frequencies RG-8x converts 44% of the RF output to useless heat before it ever reaches the antenna. So of the maximum of 50 watts transmitted by a GMRS radio only 27 watts reaches the antenna. That’s the Forward Power. 
Then, if the impedance of the antenna isn’t perfect, some amount of the power is reflected back towards the radio. That’s the Reflected Power. By the time it reaches the radio 44% of the reflected power has been converted to heat.

So, let’s say you have an SWR meter right at the radio and let’s also assume that the antenna truly has an SWR of 10.0:2 at 462 MHz. An SWR of 10.0:1 means 67% of the forward power is reflected. That’s a pretty abysmal SWR for any antenna. So for a 50 watt transmitter 33 watts reflects off the feed point of the antenna and back towards the radio. 
But now let’s introduce attenuation. 50 watts doesn’t really reach the antenna. We’re using RG-8x so only 27 watts reaches the antenna. Two-thirds reflects  back towards the radio, so only 9 watts of power is radiated instantly. 18 watts is the reflected power at the feed point but of course 44% of that becomes heat due to attenuation. 56% of 18 watts, or 10.1 watts makes it back to the SWR meter. So the SWR meter sees all 50 watts forward power and only 10.1 watts reflected power. It’s calculation says that is a perceived SWR of about 2.6:1.

The point is that lossy coax results incorrect SWR measurements, whether done by a nanoVNA, an expensive bench model Vector Network Analyzer, or the priciest antenna analyzer out there and seeing a lower SWR becomes meaningless.  
 

Put the SWR meter at the end of the run of cable and before the antenna if possible and you will get a more true reading of the antenna SWR.  Coax can introduce as SWR as well so it'll still be higher than what the near antenna reading is.

[WRUE951]

1 hour ago, LeoG said:

Put the SWR meter at the end of the run of cable and before the antenna if possible and you will get a more true reading of the antenna SWR.  Coax can introduce as SWR as well so it'll still be higher than what the near antenna reading is.

this is why I'm a fan of VNA's to get a picture of both coax and antenna on a separate scale 

[tcp2525]

2 hours ago, WRTC928 said:

Alright, smart guy. How do you hook up an antenna without using a cable?

You simply use the correct cable, and RG8x is simply not going to cut it on UHF. Only place RG8x should be used as a compromise cable at 30MHz and below, and that's a stretch. You lost so much power using the stuff it even fooled you into thinking you have an acceptable VSWR, kinda like a dummy load. Just trying to save you heartache and money.

[WRTC928]

49 minutes ago, tcp2525 said:

You simply use the correct cable, and RG8x is simply not going to cut it on UHF. Only place RG8x should be used as a compromise cable at 30MHz and below, and that's a stretch. You lost so much power using the stuff it even fooled you into thinking you have an acceptable VSWR, kinda like a dummy load. Just trying to save you heartache and money.

So what is the right cable? "You're doing it wrong" isn't helpful information. It might make you feel superior, but it sure as hell isn't going to save me any heartache or money.

[WRTC928]

Every mag mount antenna and every NMO kit I've ever seen uses RG58. I'll bet all of you have at least one on one or more of your radios. RG58 is even more lossy than RG8X, so I'm mystified about why your installation is right and mine is wrong. If the dummy load isn't between your ears, tell me something I can use. Be Elmer, not Mr Magoo.

[marcspaz]

4 hours ago, UncleYoda said:

Maybe 2 tests, one at the radio and one with meter at antenna end of cable.  Compare the two results.

 

I've done exactly this. It was to demo real-world line loss at example frequencies.  I'll share a link if you or anyone else is interested. 

[LeoG]

48 minutes ago, WRTC928 said:

Every mag mount antenna and every NMO kit I've ever seen uses RG58. I'll bet all of you have at least one on one or more of your radios. RG58 is even more lossy than RG8X, so I'm mystified about why your installation is right and mine is wrong. If the dummy load isn't between your ears, tell me something I can use. Be Elmer, not Mr Magoo.

The one I got came with CFD200, a little better than what everything else comes with.

[SteveShannon]

4 hours ago, WRTC928 said:

It sounds like you're saying there's no point in testing. Any cable that's practical for a vehicle application will be lossy. RG58 is even worse, and that's what pretty much all mag mounts and NMO kits include. Are you suggesting we should use LMR400 for vehicle antennas? Why do you assume the antenna has an SWR of 10:1? 

I picked 10:1 as an example to demonstrate how SWR readings can be completely misleading when made through lossy cable, not as any assumption of the SWR of your antenna. Th right tests are important, but tests can be very misleading.
If you want to know the SWR of your antenna, measure it at the antenna feed point, not at the radio. If you want to see how well your cable works, SWR of antenna and cable combination isn’t really an ideal test. Test the cable with a dummy load at the end. Test the SWR of the antenna separately. And put a power meter next to the antenna and then next to the radio and compare the results to see how much power is lost. 
 

[SteveShannon]

2 hours ago, WRTC928 said:

Every mag mount antenna and every NMO kit I've ever seen uses RG58. I'll bet all of you have at least one on one or more of your radios. RG58 is even more lossy than RG8X, so I'm mystified about why your installation is right and mine is wrong. If the dummy load isn't between your ears, tell me something I can use. Be Elmer, not Mr Magoo.

RG58 is used because the dielectric is solid and doesn’t deform like foam dielectrics, but you’re right that it’s even lossier than RG8x. 
Choosing cables for a mobile installation is a juggling act. The only cables that will withstand being pinched in a door are thin and stiff because of solid dielectric materials. But they’re also very lossy. So you minimize the length as much as possible and accept that they aren’t ideal. 

[WRXL702]

OMG..... Unbelievable....

Maybe All You Folks Should Consider Talking On Your Radio Stations Instead Of Constantly Arguing, Bickering, & Discussing On How They Are Supposed To Perform Better According To Your Opinionated Logic Of The GMRS / UHF Frequency Band, & Everything Else Involved.......

Not Sure What Else To Say, But It's A Pretty Sad Rendition Of Folks That Should & Definitely Know Better With All The Knowledge That Exists On This Form.

Folks Just Need To Loose Their Attitudes & Share Their Supported Knowledge........

Thx

Bob Somers - WRXL702