My UV-5R has a better SWR on my base station antenna then my base station radio. Why?

[WSBV579]

I was curious and just started fooling around today. I hooked my UV-5R up to my base station antenna to see if I could hit a repeater I use occasionally at around 30 miles out...which it did. I was kinda shocked. So, I hooked up my SWR meter, and damn! I was getting a 1.02 and 4.8-ish watts hitting that repeater.

So, I hooked my base station radio back up and tested it. It's a Btech UV-25x2. It got 1.4 SWR at about 18 watts.

Why would a 5 watt radio have a better SWR then a 25 watt radio when using the same coax and antenna?

[SteveShannon]

3 minutes ago, WSBV579 said:

I was curious and just started fooling around today. I hooked my UV-5R up to my base station antenna to see if I could hit a repeater I use occasionally at around 30 miles out...which it did. I was kinda shocked. So, I hooked up my SWR meter, and damn! I was getting a 1.02 and 4.8-ish watts hitting that repeater.

So, I hooked my base station radio back up and tested it. It's a Btech UV-25x2. It got 1.4 SWR at about 18 watts.

Why would a 5 watt radio have a better SWR then a 25 watt radio when using the same coax and antenna?

One likely reason is that your swr/power meter does a poor job of detecting the much lower reflected power level of the five watt radio. 

[WSBV579]

2 minutes ago, Sshannon said:

One likely reason is that your swr/power meter does a poor job of detecting the much lower reflected power level of the five watt radio. 

I'm using the Surecom SW 102, that I'm sure most people use. Are they not meant to measure HTs that tx 5watts?

[SteveShannon]

6 minutes ago, WSBV579 said:

I'm using the Surecom SW 102, that I'm sure most people use. Are they not meant to measure HTs that tx 5watts?

You’d need to look at the specs, but often devices that make measurements become non-linear at the extreme end of a measurement and with an already low SWR the reflected power will be a very low percentage of a very low forward power so it might not measure accurately. 

[WSBV579]

1 minute ago, Sshannon said:

with an already low SWR the reflected power will be a very low percentage of a very low forward power so it might not measure accurately. 

Oh, alright...I think that makes sense.

Thanks

[WRKC935]

Cable loss, and power level. 

Here's the math part if you care.

Lets say that you have 3dB of loss in your feed line to the antenna.  So that means that if you are pushing 20 watts into the the cable at the bottom, 10 watts gets to the antenna.  Now that same loss happens to the reflected power. If you have 2 watts reflected, then you will only see 1 watt of reflect in the meter.

Now lets remove the 20 watt radio and hook up a 4 watt radio.  You have 4 in, and 2 at the antenna.  You then have .2watts reflect, and .1 watt (100mW) shown as reflected power.  The meter will have a hard time even seeing .1 watt of power and doesn't really have the accuracy to correctly measure that small level of power.  So it gets inaccurate.

Same thing happens with other radio services like ham when you significantly increase the power level.  A guy running 100 watts may not see a significant amount of reflect at that power level, but when he increases to 1000 watts, it shows up.  This is why when you are using a Bird watt meter to make SWR readings, you use a 1 or 5 watt slug to take the reflected power reading and not the 50 or 100 watt slug you are using for forward power.  That 100 watt slug is not going to accurately show you a few watts of reflect.  You need a slug that will properly indicate the lower power level. AND, you need to take into account the cable loss for an accurate reading.