[TowerTalk] Inverted-L SWR Problem

Fri Jan 11 03:46:25 EST 2019

Let's try to fix that awful formatting!Stoopid Yahoo Mail!
Hi, gang,
Need some expert advice.

I have been having trouble with getting my remote SGC autocoupler to "tune" my inverted-L. It worked GREAT last season...but not this year...
After much farting around, the SG-231 now easily tunes a 50-foot inverted-L wire on 80-10 (but not 160 as before. I suspect a bad relay or relay solder joint), and a separate quarter-wave inverted-L wire for 160 has its own coaxial feed line running back to the shack (no tuner, cuz the '231 still won't match it on 160, even when it's resonant). Both antenna wires (separated by 6-10 feet in the vertical run, with each "horizontal portion" trailing away from the feed point in opposite directions) share a common radial field, 32, 45-foot-long ground radials. Each coax run is 110-130 feet of RG-6, and each is terminated at the antenna end in a choke made from 13 turns of RG-6 on FT-240-31 (or 43) cores.

This makes for a quiet, good-performing low-band antenna on the cheap. Well, it did until my present issues with the 160-m wire's SWR... I tested the coax run from the tuner to the shack for line loss with a power meter and the VNA, and the loss is low, especially at 160). I have used RG-6 for dozens of HF antennas over the past 30 years with excellent results. This is the first oddity.
After installing the 160-m quarter-wave wire, I quickly brought my MiniVNA (and its Android tablet) to the feed point to sweep for impedance and SWR. Instead of having a normal, single-line trace, the SWR and impedance traces were dynamic squiggles -- a lot like an AM modulation envelope on a scope. And although the impedances looked good at 50-100 ohms, the "SWR squiggle" seemed to suggest a low-SWR value of 3:1 or 4:1 at best, dipping at around 1.87 MHz. I got similar results when using the VNA in the shack.

(Also, each time I run a sweep when the unit is connected to the antenna, the traces change more than a little bit. I suspect something's weird, or that external RF is getting into the unit.)

I have had issues with this VNA over the years and had to resolder an internal connector at one time. I don't know if that issue has returned, or whether the device is being bothered by a 10-kW AM transmitter 3 miles away (KOLM 1520). To get another perspective I brought the KX3 to the antenna feed point (chilly at 15 F with a stiff breeze!), which produced results that are close to expected. See listing below. But in the shack (on the far end of 110-130 feet of RG-6), the high SWR readings persist and the values don't seem to track those taken with the KX3 at the feed point.

FREQ     Feed Point SWR     SWR from Shack

1.725    3.8    3.2
1.750    2.8    3.2
1.775    2.3    3.1

1.800    1.9    3.0
1.825    1.6    2.9
1.850    1.6    2.8
1.875    1.8    2.7
1.900    2.2    2.7
1.925    2.7    2.9
1.950    3.2    3.0
1.975    4.0    3.1
2.000    4.6    3.1

The 2:1 SWR bandwidth seems a bit broad, even with the rig right at the feed point, but there is a 50-foot wire and a couple trees nearby, so that's probably why. But the SWR from inside the shack really seems suspect. The "dip" is shallow and very broad...and it doesn't correlate well. I am going to replace the shack-side coax connector and adapter to see if that changes anything. Could be the cable, too. Just don't know...

Any ideas?
Thanks,
--Kirk in MN My book, "Stealth Amateur Radio," is now available from www.stealthamateur.com and on the Amazon Kindle (soon) 

    On Friday, January 11, 2019 2:42 AM, Kirk Kleinschmidt via TowerTalk <towertalk at contesting.com> wrote:

 Hi, gang,
Need some expert advice.

I have been having trouble with getting my remote SGC autocoupler to "tune" my inverted-L. It worked GREAT last season...but not this year...
After much farting around, the SG-231 now easily tunes a 50-foot inverted-L wire on 80-10 (but not 160 as before. I suspect a bad relay or relay solder joint), and a separate quarter-wave inverted-L wire for 160 has its own coaxial feed line running back to the shack (no tuner, cuz the '231 still won't match it on 160, even when it's resonant). Both antenna wires (separated by 6-10 feet in the vertical run, with each "horizontal portion" trailing away from the feed point in opposite directions) share a common radial field, 32, 45-foot-long ground radials. Each coax run is 110-130 feet of RG-6, and each is terminated at the antenna end in a choke made from 13 turns of RG-6 on FT-240-31 (or 43) cores.
This makes for a quiet, good-performing low-band antenna on the cheap. Well, it did until my present issues with the 160-m wire's SWR... I tested the coax run from the tuner to the shack for line loss with a power meter and the VNA, and the loss is low, especially at 160). I have used RG-6 for dozens of HF antennas over the past 30 years with excellent results. This is the first oddity.

After installing the 160-m quarter-wave wire, I quickly brought my MiniVNA (and its Android tablet) to the feed point to sweep for impedance and SWR. Instead of having a normal, single-line trace, the SWR and impedance traces were dynamic squiggles -- a lot like an AM modulation envelope on a scope. And although the impedances looked good at 50-100 ohms, the "SWR squiggle" seemed to suggest a low-SWR value of 3:1 or 4:1 at best, dipping at around 1.87 MHz. I got similar results when using the VNA in the shack.

(Also, each time I run a sweep when the unit is connected to the antenna, the traces change more than a little bit. I suspect something's weird, or that external RF is getting into the unit.)

I have had issues with this VNA over the years and had to resolder an internal connector at one time. I don't know if that issue has returned, or whether the device is being bothered by a 10-kW AM transmitter 3 miles away (KOLM 1520). To get another perspective I brought the KX3 to the antenna feed point (chilly at 15 F with a stiff breeze!), which produced results that are close to expected. See listing below. But in the shack (on the far end of 110-130 feet of RG-6), the high SWR readings persist and the values don't seem to track those taken with the KX3 at the feed point.
FREQ     Feed Point SWR     SWR from Shack

1.725    3.8    3.21.750    2.8    3.21.775    2.3    3.1
1.800    1.9    3.01.825    1.6    2.91.850    1.6    2.81.875    1.8    2.71.900    2.2    2.71.925    2.7    2.91.950    3.2    3.01.975    4.0    3.12.000    4.6    3.1
The 2:1 SWR bandwidth seems a bit broad, even with the rig right at the feed point, but there is a 50-foot wire and a couple trees nearby, so that's probably why. But the SWR from inside the shack really seems suspect. The "dip" is shallow and very broad...and it doesn't correlate well. I am going to replace the shack-side coax connector and adapter to see if that changes anything. Could be the cable, too. Just don't know...
Any ideas?
Thanks,
--Kirk in MN

My book, "Stealth Amateur Radio," is now available from www.stealthamateur.com and on the Amazon Kindle (soon)
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