Coax quality effects on SWR

[yqtszhj]

I’m relatively new to antenna tuning and am wondering about coax quality effects on SWR. Here is my situation.

My setup:

I picked up a Midland MXT 500 along with a Midland MXTA26 antenna. They are both good in the original configuration and I get good range to a HT. SWR is 1.01 on GMRS low channels and 1.38 on higher power channels?  Next I picked up a base antenna for the house with 25 ft. of LMR-400 cable and get 1.03 on low and 1.48 on high channels. I’m good with that.

Here is where I start chasing the rainbows:

In the pursuit of more actual power actually being transmitted, and since the Midland magnet mount antenna uses 20 ft. of some “high loss” RG-58, I have the urge to use some better cable and ditch the magnet mount since my truck would permit using a 10 ft. run of LMR-400 with very minimal bending.

My issue:

I did some testing of the Midland MXTA26 antenna using the 25 ft. of LMR-400 that works well with the base antenna, but I’m getting a SWR of 2.8 with the Midland Antenna and a one piece UHF to NMO adapter used for a Jeep mount setup. I did use a 12 inch disk as a pseudo ground plane but it made no difference one way or the other.

The question???

Would lower loss cable cause the SWR to increase from 1.01 to 2.8 requiring the antenna to be tuned a bit. The only thing I cannot test on something else is the UHF/NMO adapter but there is nothing to that and everything ohms out good. Id hate to do any antenna modification if it won’t work anyway.

 

[gman1971]

High loss cable would act more like a dummy load, eating the radio power and the reflected power as well, so it looks like you have a better antenna than it is.

You also need to measure impedance, not just SWR, and check the cable patch alone, to see the loss and the impedance of just the bare cable. Connectors could also be problematic. A bad connector can ruin the entire setup as well.

G.

[yqtszhj]

Thanks. Yeah, I don’t have anything to measure impedance at the moment. I’ll have to try and come up with something. That’s something on my I need to get list. I figured higher loss cable may dissipate some of the reflected power but needed a second opinion. Thanks.

[WRKC935]

1 hour ago, yqtszhj said:

 I figured higher loss cable may dissipate some of the reflected power but needed a second opinion. Thanks.

Uhhh NO.  This will not work for what you are trying to accomplish and in reality will do the exact opposite.

If you have a cable that is low loss then more signal gets to the antenna to be radiated.  A higher loss cable means LESS signal to teh antenna.  It's true that a higher loss cable will SHOW less SWR (reflected power) on a watt meter, the mismatch still exists at the antenna and the reflected power is still there, it's just not making it back the cable to be shown on the meter you are testing with.  In truth a high enough loss run of cable with a shorted end will show zero reflected power simply because none of it makes it back to the meter even though there is a 100% reflect or ALL the transmitted power.

Now, all that being said.  A 1.5 SWR is not that big of a deal and you are loosing so little overall power that even if that issue was corrected and you were at a 1 to 1 match, the performance of the radio and it's ability to transmit and receive would not have any noticeable change.  Frankly your grasping at straws. 

The 2.38 match is concerning however.  Not sure which Midland antenna that you are using, but I am wondering if it's a UHF antenna that is sold for ALL of the UHF band including ham and it's simply too long.  I would advise you to look at the lower frequency channels and teh higher frequency channels.  If the SMR on the lower frequency channels is lower than the higher frequency channels then the whip on the antenna is TOO long and needs trimmed.  Consult the instructions on the antenna and see if it needs cut to a certain length for the frequency range you intend to operate it on. 

[yqtszhj]

Thanks. That’s info I was looking for. I’ll check that out. It’s the Midland MXTA26 and it didn’t come with any length instructions in the package but I’m sure I can find them online. I didn’t adjust anything initially for the antenna used on factory supplied mag mount that came with the radio that used RG-58 because the SWR was 1.01 and it functioned OK. Based on the factory cable specs it calculates to losing about 2.7 dB over the length of the cable. It was then I tried some better LMR-400 cable and that was when the SWR jumped to 2.3 for the midland antenna. I’ll be using it to transmit on GMRS frequencies only so it may indeed be too long and need some trimming so I’ll look into that. Thanks again.

[Lscott]

On 3/9/2022 at 9:51 PM, yqtszhj said:

The question???

Would lower loss cable cause the SWR to increase from 1.01 to 2.8 requiring the antenna to be tuned a bit. The only thing I cannot test on something else is the UHF/NMO adapter but there is nothing to that and everything ohms out good. Id hate to do any antenna modification if it won’t work anyway.

 

I can't say the lower loss coax will cause that huge of a difference but it does make a difference.

I've use some crappy higher loss RG-58 on UHF because it results in a bit lower SWR as measured at the radio's end of the cable when using a antenna a bit out of it's optimal range. The logic for how that happens is some of the power from the radio is lost in the coax as the energy propagates from one end to the other. When the antenna is not perfectly tuned some of that power is reflected back to the radio. Of course some of that reflected power is also lost in the cable. So the power reflected back, and measured at the radio's end, is lower than what you get with a lower loss cable. The calculated SWR would thus be lower. This is true because the miss-tune antenna reflects the same ratio of the forward power back towards the radio in either case. With the high loss cable you simply get less reflected power.

[gman1971]

There are multiple things at play here, right? Without knowing the cable impedance and the actual cable loss (not the SWR), you really can't tell what is going on. I recommend getting a NanoVNA, ideally with N connectors, even though the SMA one will works just fine, but those SMA tend to wear out if you use them too much... anyhow.

The cable length will change the insertion loss, a longer cable has more loss, shorter cable less loss, etc, but varying the length will also alter the SWR readings by a hair, depending on the frequency, along with varying the impedance by a hair as well. Not by much tho.

Once you know the cable insertion loss, the impedance and the SWR for that given patch of cable, then you need to tune the antenna for 50 Ohms, and not for the best SWR, and so long that at 50 Ohms the SWR is better than 2:1, it should perform well. If the antenna is not tunable, then you need a new antenna.

G.

[yqtszhj]

Thanks Lscott. That’s exactly what I’m seeing after some more testing today. With the LMR-400 cable I was able to get the SWR to 1.58 with some tweaking and it’s still good (actually a little better on high power) with the RG-58 cable on the factory mag mount base.

Sad note on the Midland MXTA26. As seen on multiple reviews they almost cut the whip antenna a bit short. I had to raise it to the highest level that the Allen screws would still hold it to get the 1.5 SWR.

[yqtszhj]

Thanks gman. I’ll look into that NanoVNA.