Best way to evaluate 2m/70cm coax and connection issues

[KC1LML]

My apologies for asking a question that I'm sure has been discussed multiple times, but I can't seem to find a good answer. How would I go about checking my power loss due to coax that is too long or not ideal for 2m/70cm? Also cheap, poorly fitted PL-259 connectors?

In my case, I have a 5/8 wave dual band vertical antenna on a pole above my roof. A 25' R-8X cable comes down to a grounded lightning arrestor, then a 50' R-8X cable goes to my radio. My SWR meter shows a swr of 1.0 and 36 watts on 2m. It shows a swr of .5 and 32 watts on 70cm.

For what it's worth, the radio is rated 50 watts on VHF and 40 watts on UHF. Maybe what I'm showing on my swr/power meter is typical compared to advertised power ratings? In any case, I know I'm not using the best cable and my length is probably a bit long for UHF.

Is there any way other than investing in something like a VNA or Wattmetter to check my power loss?

 

[n5ims]

RG-8X is designed for HF work at relatively low power (100 - 200 watts max generally) but at VHF and higher it's not good. It will work for short jumpers, but not for runs of any real length (say 25' or more). You have 75' between your radio and antenna so 8X is not the coax for you. At best, you're dropping 3 dB (half of your power) in the coax on that 75' run using RG-8X. Once you get to UHF that increases to 6 dB (3/4 of your power). Moving to the cheapest quality RG-8 (Belden 8237) those numbers drop to 1.7 (VHF) and 3.3 (UHF). Now the 8237 is larger but pretty easy to manipulate so most folks can easily install it properly.

There is better coax you can buy but the cost goes up and often the difficulty of installation goes up as well. LMR-400 (1.1 VHF, 2.2 UHF) is a good choice if you stick with the name brands (not the cheap eBay stuff). You can order it from many ham radio stores and some will custom make them to your specified lengths with the connectors of your choice. Just remember that regardless of what coax you select, make sure it's a name brand (Belden or Times Microwave Systems for example). There are even better types of coax on the market, but their cost Is often way out of the typical ham's budget. The coax loss calculator linked below includes some of them if you are interested.

You could try to fix your current installation by redoing the connectors with better quality ones that may be installed better, but it really won't make that much of a difference. Better coax should be your first attempt.

Down to your specific question. The simplest way to check out your coax run is using a Coax Loss Calculator ( Coax Calculator (qsl.net) ). It's free and will tell you what you need to know prior to buying the cable. There are many tools to check out your antenna system, including coax, but they aren't cheap and many will give you answers that you may not understand (they are at times complex). On the bright side, many clubs have folks that own these devices and understand how to use them properly. Joining a club and asking around will probably be your best route to get it checked out for the cost of a beer or less. Good luck!

 

[nd5y]

Is there any way other than investing in something like a VNA or Wattmetter to check my power loss?

No. The only way is to measure it directly. If you can't take the coax down that could be a problem.

If you know the brand and part number of the coax you can look up the loss on the manufacturer's data sheet then measure the lenght of the coax and calculate the loss. That is only an estimate and can't take into account damaged coax or bad or incorrectly installed connectors.

There are several web sites with coax loss calculators that can calculate loss if you know the type and length of the coax.

 

[KC1LML]

Thanks guys, that's exactly what I was looking for. I'm going to have to remove the 8X and replace it. I don't know what I was thinking. I just recently got into 2m/70cm and had some extra 8X that I use with my shortwave antennas and used it for my new VHF/UHF rig.

I do have one other question. What is the power reading measuring on a swr meter? Obviously not the power being transmitted out the antenna.

 

[devicelab]

Buy one of these. Be sure to pick up the N adapters -- but it's a really handy tool to have around. If you buy a dummy load then you can easily check the power loss of your coax.

If you do it right the first time then you don't have to worry about taking your coax down!

Amazon.com

 

[nd5y]

What is the power reading measuring on a swr meter? Obviously not the power being transmitted out the antenna.

It measures the forward or reflected power at the point in the transmission line where the meter is.
To measure how much power is getting to the antenna (and accurately measure SWR) you need to put the meter at the end of the coax at the antenna.

I wouldn't trust a $56 no name Chinese Amazon meter to be accurate.

 

[devicelab]

I wouldn't trust a $56 no name Chinese Amazon meter to be accurate.

You can go ahead and spend $750 for a Bird meter. I'll take what works for a fair price.

 

[nd5y]

You can go ahead and spend $750 for a Bird meter. I'll take what works for a fair price.

I bought a brand new Bird 43 way back when they were only about $250 and all the slugs I needed.
Real test equipment isn't cheap.
Most ham radio related videos are garbage made by "content providers" and "influencers" that don't know anything.

 

[prcguy]

Some of the Chinese meters are extremely accurate and cheap like the nanoVNA series. Two useful measurements on coax including connectors would be VSWR and thru loss. A VSWR meter or a watt meter, transmitter and a dummy load will check the coax in place for basic problems that will show up as VSWR. A companion measurement is thru loss but you usually need both ends of the coax at the measuring instrument except in the case of an HP/Agilent Scalar Network Analyzer with extension cables for the detector. VSWR and thru loss measured across a wide range like 1 to 1000MHz will give you a real good idea of your cable and connector health and a cheap nanoVNA or similar device can do that and more.

 

[KC1LML]

Anyone have any experience with the NanoVNA? I got one last year as a gift. I've only used it to check SWR across a spectrum that an antenna is supposed to be tuned to. It seems to match the antennas I tried to their advertised specs.

One thing I wondered about is how placement of the NanoVNA in the antenna run affects readings. As far as the 2m/70cm mounted on my roof, I've only been able to take readings at the lightning arrestor and at my radio. Of course I can only check my magnetic mount antennas at the PL-259.

I have seen posts about using one to check cable loss, but that requires having the cable back on the ground.

Edit - looks like prcguy has used one.

 

[AB4BF]

Buy one of these. Be sure to pick up the N adapters -- but it's a really handy tool to have around. If you buy a dummy load then you can easily check the power loss of your coax.

If you do it right the first time then you don't have to worry about taking your coax down!

Amazon.com

Those are pretty good for the price, but use Pay Pal or another payment system to insure that if you receive a defective one it will be covered.
I've been watching these... Touch screen Mini1300 Meters 0.1-1300MHz HF VHF UHF ANT SWR Antenna Analyzer 720310977709 | eBay
It will do way more than my current Comet antenna analyzer and its $150 to $170.
I've been waiting to see if anyone on ARRL QST or other websites have bought and tried them. It looks good and has a 4.3 inch screen - something I wish my Comet had...

 

[popnokick]

If you mean "evaluate" to within a gnat's eyelash / .1 mW or better and require a lab / bench calibration instrument.... then you'll need to spend the $$$. But if you want a "relative" reading to help you after you make a change to determine, "Did I make it better, or did this make it worse?" and is within a reasonable, ham hobby-grade range... you can use the sub $100 meters. And as prcguy points out (and has been written here on RR in comparisons with the big $$$ instruments) the nano-vna stands up pretty well in the accuracy dept for a very low price.

 

[devicelab]

Anyone have any experience with the NanoVNA? I got one last year as a gift. I've only used it to check SWR across a spectrum that an antenna is supposed to be tuned to. It seems to match the antennas I tried to their advertised specs.

One thing I wondered about is how placement of the NanoVNA in the antenna run affects readings. As far as the 2m/70cm mounted on my roof, I've only been able to take readings at the lightning arrestor and at my radio. Of course I can only check my magnetic mount antennas at the PL-259.

Search for prcguy or my posts in the antenna forum... we've discussed it a lot.

 

[KC1LML]

Update - after replacing my 75' run of RG-8X coax with a 50' run of Belden 9913-F7. I wasn't sure what changes I would see in terms of SWR and power readings, but this was kind of surprising to me. SWR went up a little on both 2m and 70cm to 1.3 and 1.5 respectively but is still ok. But 2m power dropped from 35W to 30W and 70cm power dropped from 30W to 20W.

These readings were taken at my radio. I know it would be better to take readings at my antenna end, but that isn't practical. It's 21 feet above my porch roof.

Is it possible power readings dropped at the radio because now more power is getting out the antenna?

In terms of performance, it's as good as before and hopefully better. It may take awhile to say for sure.

 

[tweiss3]

The SWR readings are consistent with the loss of RG-8X coax, basically instead of reflecting, its just completely lost.

As for the "power" reading, since SWR slightly increased, your power output reading would slightly decrease.

 

[KC1LML]

Do you think my SWR really increased? All else is the same except for the cable. I was thinking maybe my meter was getting tricked before by the loss in the RG-8X cable.

 

[WA0CBW]

Bad coax (lossy) reduces the actual SWR reading at the transmitter. Better coax reduces the loss of the reflected power. More of the reflected power will get to the transmitter.
BB

 

[mmckenna]

Do you think my SWR really increased? All else is the same except for the cable. I was thinking maybe my meter was getting tricked before by the loss in the RG-8X cable.

The SWR for the antenna likely didn't change.
What happened is that you are probably seeing some differences in the cable losses, which will impact what the SWR sees. If the new cable has less loss, it may see more of the power reflected and show higher SWR.
There's also some funky things that can happen at specific wavelengths along the cable.
Best way to check SWR is right at the antenna, but sounds like that is not possible.

But from your setup, it sounds like you are OK. Don't get too hung up on those numbers. If the antenna is radiating well and the radio is doing what you want, just go with it.

 

[Ubbe]

The more loss in a cable the less SWR as the coax works more as a dummy load. I checked the SWR of a drum of RG58 with the ends open and it was perfect as all signal was attenuated along the coax's length. The worse the antenna, it works more like a dummy load than letting the signal out, the less the SWR. If anything in the transmission line gets hot then it's the RF power that are converted to heat energy.

/Ubbe

 

[KC1LML]

Thanks for the helpful replies!