That whole coax length thing...

[Captain Kilowatt]

CK, can you give us some details as to how you use the analyzer when you trim a 1/4 wave jumper, and what the analyzer shows you when you find the sweet spot?

1/4 wave stubs invert the impedance,IOW a 1/4 wavelength OPEN ended coax cable will appear to have a short across the other end and vice-versa. To make a 1/4 wave stub start with the cable a little long and with one end connected to the MFJ-269 start trimming the open end. When you start to get close to the resonant freq. the impedance as shown on the meter, will sharply swing towards zero impedance and look like a dead short.

 

[W5LZ]

 

[mackmobile43]

Someone should post pictures of this t connector method. I find this fascinating.

Well I don't actually use a t-connector, I use a co-phase harness and as I tune one antenna I have a dummy load connected to the other and when I have the first antenna tuned I switch sides and tune it.

 

[Marconi]

This is all of the subject of this thread, but here is the T-connector and the dummy load idea. If you substitute the analyzer for the SWR meter then the transmitter is out too. If you read the text this routine shows a special feature in the process that allows for a little finer tuning.



Here is sort of how CK did it looking for the minimum Z.



I kind of agree with CWM, I think since we are usually working with a broadbanded antenna it might not be as important to worry about a Nat's hair measurement and if the coax is named brand I have found that they are much better at producing coax with a VF that is close to specs. If you look at the end of your coax and you see the center conductor is not set squarely in the middle of the dielectric, then you might have a problem.

This said, I usually just use the math---that is pretty close. I use 492 as the factor and not 468 which mostly used when getting close to the length for a resonant radiator.

 

[W5LZ]

And if you will do the math, you will see that 468 figure is 95% of 492. Which is saying that a typical wire, such as #14, has a velocity factor of 95%. Which if true for the normal/typical range of wire sizes used to make wire antennas.
And since there are a number of factors which determine velocity factor with wire, that is a very 'general' sort of velocity factor. It will generally get you close, and just a bit too long, which is almost always better than a bit too short. That's a "fudge factor", in case you measured just a bit wrong. Or your yardstick shrunk? (Mine has a habit of doing that.)
- 'Doc

 

[The Jerk]

When SWR changes with coax length this is a key sign that there is a mismatch between the antenna and the 50 ohm coax.

If different lengths of coax effect your SWR, you need to adjust your antenna and it just may take using the right 1/2 wavelength of coax (calculated with velocity factor) to find the real match.

This was my line of thinking...

The amp is definitely not original, as the previous owner told me it had "been burnt up a few times"...it looks like a lot of the components have been replaced at some point (either all at once or piece-meal, I don't know). So the amp is definitely not perfect by any means...most likely the PO should have scrapped it and started over with a new amp. But it works...that's all I'll say. So who knows if its tuned/matched correctly? (this would explain the SWR changing with power)

I also want to remember someone telling me the Wilson 1000/5000 aren't really a true 50 ohm antenna? And now that I think about it, that would be the reasoning behind Wilson stating the coax had to be a certain length?? (this might explain why I could never get better than a 1.3:1 match)

I probably just got very lucky with that piece of coax I used...and it fooled the meter well.

I did talk to two people last night on 38LSB...one was Daytona Beach and the other was Arkansas. So my setup seems to work...conditions were really poor last night around these parts. I'm glad this provided a good discussion...I've learned that I have way to few tools (err, should that be toys) to work on antennas!

 

[GLR]

Well I don't actually use a t-connector, I use a co-phase harness and as I tune one antenna I have a dummy load connected to the other and when I have the first antenna tuned I switch sides and tune it.

makes sense

 

[W5LZ]

Two things.
First, that 1.3:1 isn't bad by any means. Shouldn't make any practical difference in performance. 'Why' you have a 1.3:1 SWR is more important than just having it. If it's a result of using the feed line to 'tune' the antenna's input impedance, it just isn't the best way of going about it. Sometimes it's necessary, but never the 'best' way.

Second.
Unless an antenna, any antenna but mobile antennas especially, has some impedance matching device as part of it, it will never be a resonant 50 ohms impedance. That 50 ohms thingy isn't 'natural', it's just an impedance that was picked as a 'standard' a long time ago (in a galaxy far away).
- 'Doc

 

[Captain Kilowatt]

Well I don't actually use a t-connector, I use a co-phase harness and as I tune one antenna I have a dummy load connected to the other and when I have the first antenna tuned I switch sides and tune it.

Truthfully that is the best way to tune a pair of phased antennas for a perfect match as it eliminates any interaction and all the headaches associated with it. It is basically the same thing as using the Tee idea but takes the exact line you will be using into account.

 

[C W Morse]

The best question is why do some people swear they see a difference in SWR with coax length and other are sure there is none? The answer will surprise you guys that see a change with length. When SWR changes with coax length this is a key sign that there is a mismatch between the antenna and the 50 ohm coax. It indicates there are standing waves on the coax line and that's why the impedance changes with length. In this case it takes an electrical 1/2 wavelength section of coax to show the true antenna impedance.

Had the antenna been tuned at 50 ohms to match the coax, any length of coax would indicate a good SWR match because the impedance is constant anywhere along the coax. If different lengths of coax effect your SWR, you need to adjust your antenna and it just may take using the right 1/2 wavelength of coax (calculated with velocity factor) to find the real match.

I can't argue with any of this. All I am saying (for me), I never ran into any situation where coax became a reason something didn't work right. I just trimmed the antenna to length, used appropriate matching at the feedpoint (if needed), and it always worked out. I never CARED about getting everything just "so", so long as the thing worked correctly and I got good results. I've rarely had to dust off the books to find out why an antenna wouldn't "tune" right, or fiddle with feedline. Maybe been lucky for 40 years? Who knows! That's why I maybe ridiculed the coax length gurus so much? It just never became an issue for me so that I had to ASK why, or pore over coax formulas to get a system to work right! I just used 468/freq in mhz (divided by 2 for dipoles) and hung the thing! If it was "off", I trimmed it until it worked! Simpleton, me!

Correct me if I'm wrong; I'm always up for learning. 1005= full wave in free space

936= full wave a solid medium


492= Half wave in free space. Simple solutions to a simple problem!

468= Half wave in solid medium (like coax) Which, to me, takes care of all that other malarkey about velocity factor and coax length! Now, you're telling me (tongue in cheek) that the station I've used for years to talk from 2 miles away to Moscow, Russia has the "wrong" coax length! (kidding, so don't take me seriously: I'm having fun with this topic)


CWM

 

[Captain Kilowatt]

I just used 468/freq in mhz (divided by 2 for dipoles) and hung the thing! If it was "off", I trimmed it until it worked! Simpleton, me!

CWM

Don't get me wrong Jerry, I agree with MOST of what you say but I do have a couple of questions for you. If it was "off" and you trimmed the antenna how did you know when it was correct? Did you use an SWR meter or an antenna analyzer? If so where was the meter/analyzer located and how much cable was between it and the antenna? Do you disagree that if the meter was reading a good match at the shack end the match at the antenna end may in fact be way off due to the length of cable acting as an impedance transformer? I'm not disputing that rough and dirty will work because it can and I have used some pretty shitty antennas to make some pretty fantastic contacts in the past ( and probably will in the future as well ) and I never bother to use tuned feeders on my antennas EXCEPT when tuning them before placing them in service and THEN I use tuned feeders just to set the tuning and then remove it and replace the feeder with the regular coax cable to the shack.

P.S. I don't think anyone here is saying that the antenna system MUST have 1/2 wave multiples of coax in order to work but they (I) am saying that in order to truely see the actuall impedance the antenna is presenting to the line then one must use multiples of 1/2 wavelength of coax when CHECKING the match.

 

[Marconi]

Truthfully that is the best way to tune a pair of phased antennas for a perfect match as it eliminates any interaction and all the headaches associated with it. It is basically the same thing as using the Tee idea but takes the exact line you will be using into account.

CK, the t-connector procedure is probably done at times. Doing even less to install a co-phase setup is probably done more often though.

I ask why you would think it is best to remove a part of a system, from the system, in the process of tuning that system like you suggest?

The "co" part of the word co-phase means "with," so why try to tune the system "without" all the parts? Sounds like to me you are suggesting to tune a 2 part system by eliminating one part. Don't you think each antenna is affected by the presence of the other in a co-phase setup? Isn't this modest phasing effect exactly the point you are trying to accomplish---in order to see a little gain, and directionality?

Do you think you can tune your antenna to equal the same match as a good dummy load using a co-phase harness when you can't be sure that the harness is actually transforming as you would expect without tuning the harness? The harness is what needs to be correct, the antennas you use should already be similar and pre-tuned in place---tuned individually with a small jumper as close to the feed point as possible while both antennas or in their installed locations. You want each antenna's tune to reflect the affects of the other antenna.

The really big variables with co-phase setups are getting two antennas that can show a similar load under similar conditions and in this case this means with each other in the setup location, getting them just close enough together or farther enough apart so as they are not destructive to the other antenna's pattern, and most importantly that the harness is transforming the combination of the loads to as good of a match as possible measured at the TX end of the harness.

If you do the T-connector and dummy load idea you will see exactly how important the length of the test jumper is to frequency, and this is exactly what is important with a co-phase harness too, if the setup is to work even close to right.

Don't take this wrong, plenty of these systems work otherwise, but the question is how good---if they are not tuned correctly?

 

[Marconi]

Don't get me wrong Jerry, I agree with MOST of what you say but I do have a couple of questions for you. If it was "off" and you trimmed the antenna how did you know when it was correct? Did you use an SWR meter or an antenna analyzer? If so where was the meter/analyzer located and how much cable was between it and the antenna? Do you disagree that if the meter was reading a good match at the shack end the match at the antenna end may in fact be way off due to the length of cable acting as an impedance transformer? I'm not disputing that rough and dirty will work because it can and I have used some pretty shitty antennas to make some pretty fantastic contacts in the past ( and probably will in the future as well ) and I never bother to use tuned feeders on my antennas EXCEPT when tuning them before placing them in service and THEN I use tuned feeders just to set the tuning and then remove it and replace the feeder with the regular coax cable to the shack.

P.S. I don't think anyone here is saying that the antenna system MUST have 1/2 wave multiples of coax in order to work but they (I) am saying that in order to truely see the actually impedance the antenna is presenting to the line then one must use multiples of 1/2 wavelength of coax when CHECKING the match.

CM, I see you get my point about transformation in you last post. I agree with most of what you say and I think you understand as I see this subject also, but it is always best to tune antennas at the feed point when possible.

BTW, I see CWM is not cutting us much slack using 468, and he is still trimming.

 

[Captain Kilowatt]

CK, the t-connector procedure is probably done at times. Doing even less to install a co-phase setup is probably done more often though.

I ask why you would think it is best to remove a part of a system, from the system, in the process of tuning that system like you suggest?

The "co" part of the word co-phase means "with," so why try to tune the system "without" all the parts? Sounds like to me you are suggesting to tune a 2 part system by eliminating one part. Don't you think each antenna is affected by the presence of the other in a co-phase setup? Isn't this modest phasing effect exactly the point you are trying to accomplish---in order to see a little gain, and directionality?

Do you think you can tune your antenna to equal the same match as a good dummy load using a co-phase harness when you can't be sure that the harness is actually transforming as you would expect without tuning the harness? The harness is what needs to be correct, the antennas you use should already be similar and pre-tuned in place---tuned individually with a small jumper as close to the feed point as possible while both antennas or in their installed locations. You want each antenna's tune to reflect the affects of the other antenna.

The really big variables with co-phase setups are getting two antennas that can show a similar load under similar conditions and in this case this means with each other in the setup location, getting them just close enough together or farther enough apart so as they are not destructive to the other antenna's pattern, and most importantly that the harness is transforming the combination of the loads to as good of a match as possible measured at the TX end of the harness.

If you do the T-connector and dummy load idea you will see exactly how important the length of the test jumper is to frequency, and this is exactly what is important with a co-phase harness too, if the setup is to work even close to right.

Don't take this wrong, plenty of these systems work otherwise, but the question is how good---if they are not tuned correctly?

First the use of the term "co-phase" truely means nothing. Yep, that's right NOTHING. It is a made-up phrase used ONLY in the CB service and is used NO WHERE else in ANY radio service. The proper term is simply "phased". There is no "co" part of anything. You use a phasing harness to connect two antennas in (or out of ) phase. There I got that off my chest.

Second,since each antenna should be tuned for a perfect 50 ohms it does not mater if the "other" load is an antenna or a dummy load. The antennas radiation fields are what combine in or out of phase giving the pattern and the mutual effect of each antenna is extremely small unless it becomes grounded and then that is another story. It becomes a parasitic element and that's another story to deal with.

As for the phasing harness itself not being tuned, there are only two possibilities. Either buy a good one and assume it is doing what it should, which is pretty much what we do with almost everything, or build one yourself in which case you would have had to cut the lengths according to your measured and tested results. The harness would then be doing what it should and if not then you screwed up somewhere and everything done to the antenna system after that becomes moot.


As for placing a short jumper and meter at the base of each antenna do you also insert the same inline with the other antenna so as to offset the effects of the first one? If you don't you are delaying the signal more to one antenna than the other which will in turn upset the phase albeit slightly.

Either way I think we are starting to split hairs here as either method has been shown to work satisfactorily and the only definitive answer as to which method is better will have to come from using professional testing gear on an approved antenna range.

 

[BOOTY MONSTER]

just to make sure im understanding correctly .........

when yall talk about "tuning the antenna at the feed point" are you suggesting to use something like a double male coax connector between the antennas so-239 and the meters so-239 antenna connection ?

Double Male Coax Connector