Pages: [1]   Go Down

Author Topic: Which SWR to believe?  (Read 501 times)

WB7DND

  • Member
  • Posts: 16
Which SWR to believe?
« on: October 31, 2021, 10:38:02 AM »

When using a Yaesu FT-818nd and an Alex loop antenna, I have three ways to measure SWR - a Rigexpert AA 30 antenna analyzer, the internal meter on the 818nd, and an Alex tune.  (If you are unfamiliar with the Alex tune, please don't respond.). When using the Alex tune, it matches the 818nd's meter.  That is, the brightest light on the Alex tune corresponds to the lowest SWR reading - no bars - on the 818nd.   However, the Rigexpert indicates lowest SWR at a higher frequency - about 17 kHz higher.  If I transmit on the frequency indicated by the Rigexpert, I get 3 or 4 bars on the meter in the 818nd.

I seem to get about equal RBN reports either way.

Which measurement should I rely on as showing the frequency for lowest SWR?  Do I need to calibrate the Rigexpert?  If so, how is that done?

Thanks

Logged

K1VSK

  • Member
  • Posts: 1950
Re: Which SWR to believe?
« Reply #1 on: October 31, 2021, 11:40:28 AM »

What SWR does the RigExpert read and how much variation do you see over 17kHz? Assuming it is low, any variation between the bar graphs is likely irrelevant.

Sounds like you may be worried about a  problem that doesn’t exist.

Logged

K7MEM

  • Member
  • Posts: 1058
    • JavaScript Electronic Notebook
Re: Which SWR to believe?
« Reply #2 on: October 31, 2021, 11:56:04 AM »

Is it preventing you from operating?

If not, as long as the SWR is low at your operating frequency, the exact value doesn't matter. SWR meters are usually only accurate to within 5-10 percent. Probably, the most accurate of your indicators is the Regexpert. You might check the calibration, but I wouldn't go further than that.

I have a IC-735, but the internal meter is small and the FWD/REV switch is on the back of the radio. Very inconvenient. So I use a home brew SWR meter between the radio and the tuner. The meter scale has never been modified to read SWR. It just has the original 0-500 uA scale. But I know that a mid-range reading is around 3:1. Anything on the lower 1/3 of the scale is less than 2:1. So all I do is tune for the lowest reading. The tuning is usually for the center of the area where I am operating, so I don't bother re-tuning, even if I move around a bit. As long as I know my limits, I'm good.
Logged
Martin - K7MEM
http://www.k7mem.com

WB7DND

  • Member
  • Posts: 16
Re: Which SWR to believe?
« Reply #3 on: October 31, 2021, 03:28:18 PM »

If, for example, I adjust the variable capacitor on the Alex Loop to obtain the lowest SWR for 14.065 as shown by the Alex tune/818nd meter combination, the Rigexpert will show an SWR at that frequency of about 2.0.  The Rigexpert shows a lowest SWR  (about 1.2:1) at about 14.082.

On 20 meters, the Alex Loop will give a range of about 20 kHz where the SWR is 1.5 or less.  This is significant for me because it enables me to operate over a range of, say,  14.025 to 14.045 without retuning the Alex loop.  I use that frequency range in the weekly CWT sessions that CWOps puts on.  So, it is important for me to know the actual Alex loop capacitor adjustment for the lowest SWR in the middle of that range.  If I have an SWR of 2.0 at 14.035, then a significant portion of the 14.025-14.045 range will have a too high SWR.  (For the CWT sessions, I like to stay at 14.025 to 14.045.  On weekends, I like to hang out at 14.050-14.070.)

Logged

KD6VXI

  • Member
  • Posts: 1022
Re: Which SWR to believe?
« Reply #4 on: October 31, 2021, 04:12:35 PM »

Are you using the dame feeding for all measurements?

IE:  When you make your measurements with the external swr meter do you then disconnect the antenna feedline from the swr meter and connect it to the rig expert?

If not, any difference in feedline can cause differences in vswr

--Shane
KD6VXI
Logged

WB7DND

  • Member
  • Posts: 16
Re: Which SWR to believe?
« Reply #5 on: October 31, 2021, 04:41:03 PM »

Same feedline.
Logged

GRUMPY2021

  • Posts: 280
    • HomeURL
Re: Which SWR to believe?
« Reply #6 on: October 31, 2021, 06:14:50 PM »

I've heard for a long time the SWR on the 817/818 is just a token meter and probably should be ignored.   Go with the RigExpert.   
Logged

VE3WMB

  • Posts: 367
    • HomeURL
Re: Which SWR to believe?
« Reply #7 on: October 31, 2021, 08:07:00 PM »

I would go with the Alex Tune / FT-818.  It is the rig that you are trying to protect. It is more important that you tune for minimum SWR on the FT-818. The SWR bars are not very accurate but minimizing the number does mean minimizing SWR. The Alex Tune measures Field strength of the loop which will always be highest at resonance so I would believe it as well.

There is also a 3rd data point that you can use and that is band noise. Make sure that IPO is off on the FT-818 and as you tune near resonance on the Alex Loop you should hear a very significant peak in noise. I can usually manage to tune my Alex Loop to better than 1.5:1 SWR just by ear.

Cheers

Michael VE3WMB
Logged

N7EKU

  • Member
  • Posts: 1471
Re: Which SWR to believe?
« Reply #8 on: November 01, 2021, 08:19:52 AM »

This ^  ^  ^

Likely the tuning is somewhat influenced by the ground plane (mass of metal) of the rig, when you change that by changing to the Rigexpert, the tuning is being affected and it is then being tuned to the wrong point.

In addition, you could easily check the SWR meter on the rig using some fixed resistors and transmitting CW at low power.

73.
Logged
Mark -- N7EKU/VE3

W9IQ

  • Member
  • Posts: 8866
Re: Which SWR to believe?
« Reply #9 on: November 01, 2021, 08:34:48 AM »

This ^  ^  ^

Likely the tuning is somewhat influenced by the ground plane (mass of metal) of the rig, when you change that by changing to the Rigexpert, the tuning is being affected and it is then being tuned to the wrong point.

In addition, you could easily check the SWR meter on the rig using some fixed resistors and transmitting CW at low power.

73.

If that is happening then a proper common mode choke or two is a likely solution. The near-field of a small loop antenna can readily induce common mode current on the shield of the coax. The matching unit can also promote common mode current.

12 turns of RG-400 on an FT240-31 core will make a nice, wideband HF choke. The RG-400 can be purchased for a very reasonable price on eBay and Amazon in a short length with the ends already installed. The FT240-31 core is a Fair-Rite part number 2631803802 which can be purchased from all the major vendors. Build two - one for by the antenna and one for the rig end.

- Glenn W9IQ
« Last Edit: November 01, 2021, 08:42:57 AM by W9IQ »
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

WB0FDJ

  • Member
  • Posts: 262
Re: Which SWR to believe?
« Reply #10 on: November 02, 2021, 11:06:36 AM »

I would go with the Alex Tune / FT-818.  It is the rig that you are trying to protect. It is more important that you tune for minimum SWR on the FT-818. The SWR bars are not very accurate but minimizing the number does mean minimizing SWR. The Alex Tune measures Field strength of the loop which will always be highest at resonance so I would believe it as well.

Several years ago I read some work done by a FT-817 owner, looking at accuracy of the SWR indicator. No bars consistently indicated a low SWR and appeared reliable. The problem is when you find, say, one bar or two bars there is no direct correlation to an exact reading. For example two bars could be anything from 1.3 to 1.7 or even higher. I have found this true on my own FT-817. Zero bars is very close to 1.1. FWIW.

WB0FDJ Doc
Logged
Pages: [1]   Go Up