Significant difference between LP 700 and RF power of TG and PG

Jim Sansoterra

LP 700 shows 1300 watts (cw) and both the PG and TG show 850w +/_ 5w difference between them. LP 700 is seeing the antenna i.e. last in the RF chain. Feed line is 7/8 Heliac,375 feet and the last 65 feet is Bury-Flex. Antenna is 6-JK and band selected is 20. SWR on the 6-JK is 1.4 at the antenna. Many offers to buy "power producing tuner that runs on 12vdc" Hi Thoughts? JIM

Mike-VA3MW

Jim

Your SWR will only be the same on all measurement devices if you are testing into a 50-ohm resistive load. You are testing on an inductive load that is showing a 1.4:1 SWR, and there is nothing wrong with that SWR Reading, although it will actually be higher if you read it at the antenna.

You can read about it in this document by the ARRL. https://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf

SWR readings vary across the feedline and that is what you are seeing.

As for the power producing tuner? You can't cheat physics.

Mike va3mw es HNY

https://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf

Jim Sansoterra

Thanks, Mike..appreciate your thoughts. The 1.4 was at the antenna, at 55 feet, prior to attaching coax.. I understand the inductive--resistive issue. Root question remains---why the difference in power readings---which do I trust given the huge difference? Power producing tuner was an attempt at humor.....Regards, JIM

Mike-VA3MW

Hi Jim

Sorry I missed it. :). Sometimes you just never know.

With an SWR mismatch, your power will also be inaccurate. It is only accurate on a 50 ohm resistive load.

73

Jim Sansoterra

Thanks...will let you know if I ever figure it out. Mismatch is one thing but the entire set up is remarkably flat (for a change) Happy New Year JIM

Mike-VA3MW

It is all about standing RF waves (waves that do not move in the feedline) and constructive and destructive interference.

I was trying to find a good article on it, but I am still looking.

In college, we built a test jig at 100Mhz that had jumpers in 1/4 wave increments. (ok, I built it and I feel like I soldered about 100 connectors on all the different cables we required). We then measure the RF power along the feedline on a 50 ohm load and on a 25 ohm load. I wish I still had the notes from it. :)

Butch

Jim - Even with running into a 50 ohm RF load (Palstar DL5K) I see differences. My PG-XL and TG-XL (with latest updates) typically indicate power level indications which are fairly close (~ 30 watts difference @ 1kw), while my LP-700 shows 200-300 watts lower. ˉ\_(ツ)_/ˉ

Butch, KF4HR

David Decoons, wo2x

Jim, I know you mentioned your system is relatively flat but couplers for LP 700 should be before amp and between amp and tuner.

Using Flex TUNE button, I see about a 5% difference in the PG XL, TG XL, and high power LP700 coupler with a 2000 watt 50 Ohm load on output of tuner and tuner in bypass.

The LP700 reads substantially higher PEP as it has much faster sampling rate for power sampling. Also the latest beta firmware for LP700 (not on their site) adds input power, output power and amp gain on the Trap/waveform screen.

73

Dave wo2x