Correct Place to Test SWR?

[HomerBB]

I always try to tune a new antenna at or near as possible the antenna feedpoint. Then I check the antenna system at the radio. When I know the antenna is good I will know any differences I have at the radio are due to my feedline or other components like the power meter or tuner.
If it is good enough I quit obsessing about it. If they are bad I start trying to correct the feeding system issues. If it's too good I suspect I have a terrible coax with bazillion dibblies of losses.

 

[Riverman]

Okay, have my plumber's butt showing and here we go.
Finally took some readings at the antenna (with 3' jumper) and adjusted the tuning rings accordingly.

At the Antenna:
CH 1 SWR = 1.22
CH 20 SWR = 1.08 R= 47.0 X = -0.2
CH 40 SWR = 1.19

At the Meter:
CH 1 SWR = 1.32
CH 20 SWR = 1.31 R= 63.7 X = -6.8
CH 40 SWR = 1.33

At the Amp:
CH 1 SWR = 1.34
CH 20 SWR = 1.23 R= 47.9 X = -10.0
CH 40 SWR = 1.19

At the Radio:
CH 1 SWR= 1.12
CH 20 SWR = 1.23 R = 56.1 X = 4.4
CH 40 SWR = 1.39

Could be some CMC although it is not readily present as the readings are not affected by my touching any of the equipment and are steady with only the slightest of fluctuations.

The slightly higher readings indoors at the Meter are likely due to losses in the 87' of RG8X. And the amp is influencing the other readings. That would be my guess.

Since there are no bad readings in the bunch, am going to follow Homer's lead and forget it and keep on trucking. What say ye?

 

[314]

Was taught back early 90's, Test swr with Analyzer @ the antenna with coax your gonna use to the radio, 18ft 35ft..etc. Adding wattmeter, amp @ the radio will of course change things @ the shack a little or a lot, but should not change what's @ the antenna, if tuned right. I've went commando since the 90's, no meter inline unless I'm setting dead key or seeing power out but never keep it inline.

 

[M0GVZ]

Unable to test at the antenna feedpoint, so which of these are impostors?

1. At the SWR Meter: SWR = 1.44 R = 71.5 X = -3.6
2. At the Amp: SWR = 1.37 R = 49.5 X = 15.7
3. At the Radio: SWR = 1.05 R = 50.6 X = 2.2

All of them. Only time the first bit in the chain from the antenna isn't an imposter is if the coax from the antenna to the end you connect to is multiples of an electrical half wave long.

Assuming: Radio -> Amp -> SWR meter the radio is only measuring the impedance of the input of the amplifier, not the antenna.

Could be some CMC

Unless you've an adequate RF ground and have used a RF choke that's a given.

 

[Riverman]

All of them. Only time the first bit in the chain from the antenna isn't an imposter is if the coax from the antenna to the end you connect to is multiples of an electrical half wave long.

Assuming: Radio -> Amp -> SWR meter the radio is only measuring the impedance of the input of the amplifier, not the antenna.


Unless you've an adequate RF ground and have used a RF choke that's a given.

How about the readings taken 3' from the Antenna?

Concerning CMC, am using five #31 snap-on ferrite beads/cores at the entrance to the shack.

 

[Riverman]

Okay, FINALLY got this through my thick skull.

Measure SWR at the antenna. Then, after line loss, impedance mismatches and whatever else is going on between it and the radio itself, the power that does make it to the antenna goes forward or is reflected according to THAT reading. An accurate resonance measurement resides at the same location.

Realized earlier that this thread is pretty much a duplicate of an ongoing one titled Power Loss at Antenna. Sorry.

Over and out.

 

[Slowmover]

Okay, FINALLY got this through my thick skull.

Measure SWR at the antenna. Then, after line loss, impedance mismatches and whatever else is going on between it and the radio itself, the power that does make it to the antenna goes forward or is reflected according to THAT reading. An accurate resonance measurement resides at the same location.

Realized earlier that this thread is pretty much a duplicate of an ongoing one titled Power Loss at Antenna. Sorry.

Over and out.

Cut it out.

(Same feelings here; the below is NOT addressed to you specifically, but to that ME we all share feels this way. Not just this subject, but any. Do the details of the knot-tied count for more than determination to master it? )

You got your Golden Years Pass at the same discount as the rest of us codgers.

Do I “wish” I was again 14-yrs old and properly-motivated? Yup.

But it won’t happen that way.

So, be glad Daddy ain’t moving the family and your teenaged butt over to McKinney, TX to work at the Raytheon Electronic Test Range. Family & peer pressure might be a little high in that case . . . and un-ending decades stretching out ahead where this ain’t your strength (maybe as you see it) to always be bench-warmer.

That is how I see it.

Planted a sloped dipole in the front yard on a push-up? LED-adorned? Neighbors who might want to know something about communication without a corporate intercessor could very well be grateful on where to start.

IMO, that’s where to put that energy. I bug truck drivers as I can. A list. A contact. I don’t know squat HERE, but I’ve got a good idea of where to go for starters.

If that isn’t reason enough for the site, I don’t know what else would serve so well. It ain’t about me (hard to believe, I realize).

Thousands in a country of tens upon tens of millions.
(Something beats nothing).

Do what we can, while we can.

(And this thread was one more where a puzzle-piece fitted. Thanks).

By the way, that test range has been hosting U-foes for awhile.
The buried tech now coming out as camera images and USN patents.

Which means A WHOLE BUNCH OF “smart guys” are about to get official word: they ain’t.

The 2004 USS Nimitz “incident” is the fun one to read. Pacific Test Range.
Read the Metallicman article on it.

Same in the study of history.
U-foes don’t add up to a dot on those page revisions. False science and false history are cousins.

Maybe this is the right place, eh?

Get the doorbell to work.
That’s one thing.

Being equipped to answer the door is another.

Co-operation built our society. Voluntarily. Not coerced.

(Polish halo. Add grey to hair. Obtain fine ash cane. Flannel shirt year-round. Something always wrong with appearance. Button or shirttail. Practice now while it’s E-Z)

When they send you off with encomiums, it’s because you recognized a new world was being born.

It’s always thus.

.

 

[Riverman]

Cut it out.

(Same feelings here; the below is NOT addressed to you specifically, but to that ME we all share feels this way. Not just this subject, but any. Do the details of the knot-tied count for more than determination to master it? )

You got your Golden Years Pass at the same discount as the rest of us codgers.

Do I “wish” I was again 14-yrs old and properly-motivated? Yup.

But it won’t happen that way.

So, be glad Daddy ain’t moving the family and your teenaged butt over to McKinney, TX to work at the Raytheon Electronic Test Range. Family & peer pressure might be a little high in that case . . . and un-ending decades stretching out ahead where this ain’t your strength (maybe as you see it) to always be bench-warmer.

That is how I see it.

Planted a sloped dipole in the front yard on a push-up? LED-adorned? Neighbors who might want to know something about communication without a corporate intercessor could very well be grateful on where to start.

IMO, that’s where to put that energy. I bug truck drivers as I can. A list. A contact. I don’t know squat HERE, but I’ve got a good idea of where to go for starters.

If that isn’t reason enough for the site, I don’t know what else would serve so well. It ain’t about me (hard to believe, I realize).

Thousands in a country of tens upon tens of millions.
(Something beats nothing).

Do what we can, while we can.

(And this thread was one more where a puzzle-piece fitted. Thanks).

By the way, that test range has been hosting U-foes for awhile.
The buried tech now coming out as camera images and USN patents.

Which means A WHOLE BUNCH OF “smart guys” are about to get official word: they ain’t.

The 2004 USS Nimitz “incident” is the fun one to read. Pacific Test Range.
Read the Metallicman article on it.

Same in the study of history.
U-foes don’t add up to a dot on those page revisions. False science and false history are cousins.

Maybe this is the right place, eh?

Get the doorbell to work.
That’s one thing.

Being equipped to answer the door is another.

Co-operation built our society. Voluntarily. Not coerced.

(Polish halo. Add grey to hair. Obtain fine ash cane. Flannel shirt year-round. Something always wrong with appearance. Button or shirttail. Practice now while it’s E-Z)

When they send you off with encomiums, it’s because you recognized a new world was being born.

It’s always thus.

.

Feel better already!

 

[bob85]

When you put the choke at the shack entrance you still have the coax from choke to feedpoint that could carry cmc even if your choke was a magic milliom ohm none reactive choke,
you just made a BIG gainmaster in disguise,

the choke should be at the feed-point on 5/8 groundplanes,
or about 1/4wave down from the feed-point on stardusters & about 1/2wave from feed-point on astroplanes,

right next to the isolator in your mast

 

[Riverman]

When you put the choke at the shack entrance you still have the coax from choke to feedpoint that could carry cmc even if your choke was a magic milliom ohm none reactive choke,
you just made a BIG gainmaster in disguise,

the choke should be at the feed-point on 5/8 groundplanes,
or about 1/4wave down from the feed-point on stardusters & about 1/2wave from feed-point on astroplanes,

right next to the isolator in your mast

Woopsidaisy.

10-4.

 

[M0GVZ]

How about the readings taken 3' from the Antenna?

They'll be slightly out but far more accurate.

Concerning CMC, am using five #31 snap-on ferrite beads/cores at the entrance to the shack.

They're in the wrong place. They should be at the antenna socket end of the coax or as near as possible. As it is a percentage of your transmitted RF will be being spewed out randomly from the coax and not from the antenna where we want it to be spat out from because whilst you'll be choking common mode from the entrance of the shack into the shack you'll still have it on the rest of the coax to the antenna.

 

[M0GVZ]

Woopsidaisy.

10-4.

Don't beat yourself up about it. When I was into CB radio before becoming a ham I didn't even do as much as you. Internet didn't exist so any knowledge you got was from the locals and as I know now 99% of it was pure bullshit. It's only when I got my ham licence, got into antenna design and experimentation and spent lots of time over months and years learning about it that I learned this stuff.

As far as I was concerned before learning that, and indeed how many hams who don't bother to learn about antennas still are too, I'd put up an antenna on a pole, connect it to the radio and I'd make contacts and be happy. I did from time to time suffer issues from common mode RFI but didn't know that was it, instead just assuming noise was noise and distorted audio was either me running the mike gain too hard or the radio/PSU having a problem. I was talking to people, they were talking to me so all good right?

When I got into mobile HF operating I learned a whole bunch more I didn't know there either. Installing a CB or HF amateur set up as per the advice at www.k0bg.com takes a lot of time, typically at least a full day or more, but the results are worth it. I can go out in the countryside away from man-made noise, sit there with the engine running on the 10 or 11m band and have S0 on the meter, even with a pre-amp on. Any noise I do get is either from solar conditions but mostly passing cars. 30 miles with 4W is what I aim for as being acceptable distance wise from a mobile install (as long as hills etc don't get in the way) and I've had 30 mile mobile to mobile contacts with a friend also running 4W. I always get into the next town 10 miles away mobile to base with a S9 report.

 

[Marconi]

Correct Place to Test SWR?

Question? Shouldn't these guys also be considering Common Mode Current issues in their thinking about where to test their SWR?

When I start a model I typically don't add a feed line. So, if the mast shows signs of currents I assume the antenna is producing CMC's, and if I add a feed line the model will shows common mode currents. So, I then consider that no length of feed line will act transparent, and the numbers will change with changes in feed line length.

When I can add a choke to the top of the radiator, at the feed point and it works to help eliminate these currents...then the feed line becomes transparent as we should expect. Then with changes in length I only see the minor effects of coax losses. The feed line becomes transparent...showing little or no changes to the numbers no matter the length.

The note above is to a friend. I asked a question and briefly explained my thinking on the issues posted in this thread.

Below, I made the models of a horizontal 1/2 wave dipole as a simple example to try and demonstrate the importance of mitigating Common Mode Currents (CMC) on an antenna in order to make the feed line transparent when measuring your SWR, any where, regardless of feed line length. Transparent means, "...a feed line having little or no ill-effects by its presence in the air or in a model. The only real difference we should see when adding a transparent feed line is due to the small effects of coax resistance and length.

Models and notations below:

1. Free Space model showing Average Gain = 1 which indicates a reliable model noted at the bottom of the 1st page.;.the Control Panel.

You will note that my FS model's match is a little off. This is because when I add the mast to the model and set the losses back on for the model the effect is capacitive, making the FS model show to be a fraction short, and I wanted the Real Earth model to be as true to a good and resonant match as possible for this demo. So, just in case someone questions the match...I left the FS model as is.

2. is a Real Earth model. I added a mast and a 216" inch physical 1/2 wavelength Feed Line to the FS model #1. This model does not have any Common Mode mitigation for the red line currents we see on the mast. See the red line currents noted in the Antenna View, the first page for this model.

Also note the gain = 6.89 dbi at 14* degrees take off angle. We'll compare this gain to model #3 with a choke added, as noted in model #3 below. Also note the effects on the match due to NO CMC mitigation in the Source Data report, last page. The SWR is not so bad but the match is in the weeds...and this distinction is often missed when checking the SWR...even when using an antenna analyzer.

3. is the same as model #2, with the exception that I added a choke to the top of the mast right below the feed point of the radiator. Now compare the gain for model #2 at 6.89 dbi at 14* degrees vs. 7.61 dbi at 14* degrees TOA on an antenna that has the CMC's mitigated and making the Feed Line transparent. The match of this model is also very close to what we would expect with a Free Space model R = 73, X = 0 = 1.456 dbi at resonance.

Now, I can add any length of coax to the model that has little to no CMC and we will see little to no differences in match or gain. This is due to the Feed Line being transparent, having little to no currents flowing on the feed line or mast.

So, without considering the possible CMC's on your installation and mitigating those current when possible...your SWR results are likely just a guess. And, as this demonstration hopefully shows...there can be serious losses of gain in the process if CMC's are ignored.

I use to try and re-tune model #2 as an example, and after getting frustrated and thinking I was just chasing my tail...I would give up and I had no idea why. Look, I use to be able to get close, but I always wanted to get closer. I later found out I was fighting with CMC effects...and they are really hard to see.

 

[Riverman]

They'll be slightly out but far more accurate.

They're in the wrong place. They should be at the antenna socket end of the coax or as near as possible. As it is a percentage of your transmitted RF will be being spewed out randomly from the coax and not from the antenna where we want it to be spat out from because whilst you'll be choking common mode from the entrance of the shack into the shack you'll still have it on the rest of the coax to the antenna.

Okay. Moved them to antenna feedpoint and SWR went up.
Rather than return them to shack entrance, placed them 8.5’ from feedpoint (roughly 1/4 wave). SWR returned to original readings. Left it there. Antenna is an Antron 99.

 

[Riverman]

One last question, hopefully.
Please review the readings in my Post #17 and explain the following.
Using my Daiwa 901HP, my PEP readings are as follows:

CH 1 - 55 watts
CH 20 - 45 watts
CH 40 - 35 watts

I know they are likely not 100% accurate, but why are they not more consistent?