what is your vswr at the antenna?

[bob85]

i think waverider misinterpreted what the guy meant by "radiated power eventually evens out to 91w after about 5 cycles" as meaning it took 5 cycles for the wave to be re-reflected back in phase with the incident wave,

he only shows us the first cycle 88.91 W and the result after about 5 cycles
91w

if he had given us the numbers for 5 or 6 cycles we could see exactly where his 91w came from.

 

[Marconi]

Y'all are getting into the mind of God ideas when you start trying to predict what happens in a single cycle or five cycles of and RF wave.

It sounds to me like Grampa after 3 or 4 ice cold beers in quick order and on an empty stomach...he might say just about anything.

 

[wavrider]

To much information? We clearly differ on that. This information is useful in the right hands for tuning purposes, or if you suspect an issue somewhere in the antenna system.

There is no 5 cycle delay on reflections as the above video I linked to demonstrates, the reflection happens immediately. I wonder if they are referring to something else, perhaps that is the delay as the signal travels back and forth on the feedline in this specific case? Aside from the loss characteristics I know nothing about the feedline, including length and velocity factor.


The DB

DB as stated the post is a copy and past from the article.

IF you differ with any information in the article I do think the authors email address is also in the article.

 

[wavrider]

Y'all are getting into the mind of God ideas when you start trying to predict what happens in a single cycle or five cycles of and RF wave.

It sounds to me like Grampa after 3 or 4 ice cold beers in quick order and on an empty stomach...he might say just about anything.

Exactly Marconi,

The ONLY reason I posted in this thread is to steer the discussion to the antenna system as a whole instead of just the antenna.

What happens in every cycle is important, and is great info to help understand how the antenna system functions.

It seems like this thread is starting to deteriorate when the posts want to start nit picking an article referenced.

This forum is not the ZED.

 

[The DB]

DB as stated the post is a copy and past from the article.

IF you differ with any information in the article I do think the authors email address is also in the article.

I don't differ with the information posted, I differ with your idea that:

This is far too much information

The DB

 

[The DB]

The ONLY reason I posted in this thread is to steer the discussion to the antenna system as a whole instead of just the antenna.

First off, this thread was never about the antenna itself, it was clearly about the feedline between the radio and the antenna. That is, as you mentioned earlier, part of the "WHOLE antenna system".

The WHOLE antenna system needs to present a 50 ohm impedance to the transmitter.

It is you who is making the claim to look at the whole system, then effectively narrow down the whole system to one reading, namely the SWR reading at the radio. What you suggested is actually NOT looking at the system as a whole but simply one result from said whole system. There is a big difference between the two of those.

With one of those you don't really know what is going on with the antenna system as a whole, you simply know that the radio is happy, the other you know things like how much power actually makes it to the antenna and other factors, factors which are very relevant to the thread as a whole, and are not all directly related of the antenna itself.

Ironically, with many of your other points I agree with you. You were preaching to the quire there, for the most part. In the end, I agree, for the radios sake a low SWR at the radio is important. That being said, I am not going to devalue the rest of the information that makes up the complete antenna system in favor of that one fact. Putting the information you have in perspective is key here, and that is where my argument with your position comes from.

If you wish to limit yourself to a single reading and hope it is an accurate measure of the entire antenna system as a whole (which it really isn't), its your antenna, go for it. I, on the other hand, would like to know when things like 30% of the transmitted power is actually reaching the antenna, so when there is such a weak spot in the antenna system I can fix it.

It seems like this thread is starting to deteriorate when the posts want to start nit picking an article referenced.

My argument with that article was based on the very poor wording of two columns in a chart at the top of said article. The article itself was actually very good as it lines up almost perfectly with what I know about antenna theory. Actually, I don't think said article on the whole agrees with the point you were trying to make. I am under the impression you linked it only for the SWR reflection chart it had when you made your 5:1 SWR comment, which I then challenged, and you effectively retracted.


The DB

 

[wavrider]

op:op:op:

 

[HomerBB]

. . . The article itself was actually very good as it lines up almost perfectly with what I know about antenna theory . .


The DB

Now this is what it's all about. :laugh:

For the sake of perspective -

for my job there is a metric which I am required to assist in meeting. This metric is a result of certain operational processes that managers other than me actually engage. My job is to monitor and drive the activity through assistance and awareness, and to complete a segment of the process unique to my team.

Here's the thing, that metric must be at or below the number 4.
There is a process that can not be ignored to get it there, yet, when the contribution that my team makes to help to obtain that result was NOT DONE for one week the number came in at a 2.36. The following week, however, when our part of the process was re-engaged the metric landed at a terrible 7.1.

The point is that at one point in time the number looked good, but wasn't, and getting a little farther into the process revealed how bad the reality actually was, and how false the previous number was. It looked good, but wasn't.

So, I tune my antennas with my MFJ-259b on the operational frequency of choice at or near - e1/2ƛ jumper - and leave it there. Then I hook up my feedline and hope the radio sees something it likes. As some have alluded to in the thread, the feedline usually makes a contribution to that reading at the radio end. In most cases I see a LOW SWR at the radio end, more wide-banded than at the antenna, and with the center frequency slightly away from where the reading at the antenna showed it to be.
I have attributed this to feedline losses, and some attenuation.

My MFJ-259b will tell me what it believes a given length of coax has in losses, but the use of a dummy load and two readings with a power meter has done that for me, too. And it's easy to do - one reading has the power out from the radio, say 10 watts DK, with the power meter between the radio and the coax which has the dummy load at the other end of the coax, and then a second reading of the power meter with the meter at the far end of the coax from the radio with the dummy load directly attached to the meter.
BTW, a 100' piece of Radio Shack RG58 showed a 45% loss of power, a drop of 10 watts to 5.5 watts in one such test.

Of course, the discussion has not centered on power loss as a measure of SWR, and in my coax loss test the SWR did not change no matter whether it was read from either end of the coax from the radio/dummy load while taking the power readings. The load remained 50 OHMS and the 100' coax was not a multiple of electrical 1/2 waves.

I will have to go re-read all the named sources supplied to discuss this more intelligently, but as everyone here knows, I am an empiricist in matters pertaining to proof of theory.

What I have seen is coax losses contribute to low SWR readings on the meter at the radio end of the system. What I know about this is my radio will put out full power when it sees this low reading and it doesn't care how much of that power is actually radiated by the antenna system - it's job is done.

I have used a well made, center loaded mobile antenna on the roof of my SUV and seen a good low SWR. I have replaced it with a straight, non-loaded fiberglass antenna and also seen a good low SWR on the same center frequency. I have replaced both of those with a bottom load antenna in the same puck and seen a similar low SWR with it, too. I also have done the same with a top load firestick type antenna. I have talked locally and skip with them all. Here is what I have to decide, which is the most efficient, with gives me a system that transmits the most power, aka signal, out to my destination receiver . . .
There in lies the value of this thread.

Which one I run is determined by how much my wife fuses about how redneck my SUV looks. When she isn't looking I usually run the one that switching out gets the most responses back to my calls on a given day, and most often that is the top load.

Is it the most efficient? My scientific answer is - beats me, but if it is doing the best job for me today I am like my radio, I don't care, my job is being done.

And, BTW, I like the theory that supports my predisposition, too. . .

Hello, everyone.

 

[bob85]

its easy to measure one cycle with a tdr eddie,

i posted the pingpong effect from other sources on this forum years ago more than once,
im not saying it is correct as there is much argument over the conjugate match amongst educated people,

i am saying that what waverider claimed the article said about it taking 5 cycles to get 180 degree phase shift and reflected power re-reflected in phase with the incident wave is not what the article says if you read the whole article.

Y'all are getting into the mind of God ideas when you start trying to predict what happens in a single cycle or five cycles of and RF wave.

It sounds to me like Grampa after 3 or 4 ice cold beers in quick order and on an empty stomach...he might say just about anything.

 

[Marconi]

I ordered one on special, buy one get one free deal, and get a free coffee mug.

Megger CFL535F - TDR Time Domain Reflectometer / Cable Fault Locator

 

[bob85]

you would need something more sofisticated than that one eddie, fdr capable & more expensive.

 

[Marconi]

you would need something more sofisticated than that one eddie, fdr capable & more expensive.

Bob I agree that such testing might be possible, but I guess your comment sorta' supports my point a little bit, and that was the reason for my tounge in cheek comment relating to CB work...such testing at the cycle level will not be nearly as easy as you suggested, right?

 

[jazzsinger]

I say this because I can change the length of 50 ohm coax and the resistive load at the end of the cable and as long as the cable is 50 ohms, the MFJ was able to accurately read the value of the carbon resistor even when that resistor was 100 ohms.

Hi Donald,

Can't help wondering if you'd see the same result with a 100 ohm load that contained elements of capacitive or inductive reactance as opposed to a purely resistive 100 ohm load. Would an antenna with a 2:1 swr composing reactance show the same on any random 50 ohm cable length?

Sadly i no longer have the facilities to check it myself, but its got me curious being a firm believer in using halfwave multiples as they have always served me well, not to mention Sirio supply exactly that on their mag145pl, which by my calculations are tuned for 27.500 exactly, and the antennas they are intended for are specifically marketed towards dx'ers.

It's got me wondering why Sirio a company who have improved most antennas they've rebuilt,who have facilities you are so accustomed to Donald, why do they believe in the halfwave repeater too? or is their chosen 3.6m just sheer coincidence? I don't believe it is a coincidence.

from what i recall working on hp 4000's and especially their bases, that when i fitted random lengths of rg mini 8 or refitted plugs to cables i'd previously done for mates and they've damaged them,they would show allsorts of swr,as soon as i fitted a measured length of 1/2 wave repeater they showed the same swr curve they showed with original rg58 mil spec, both with mil-c-7f cable and later with M17/28.

Common mode current or reactance responsible? or more likely a combination of both.

I've always used 12 feet patch leads too, on mobile amps (generally zetagi b300p's) and never had any swr issues.

I ain't seen any mention of the role ground losses play in making up real world feedpoint impedance's either from anyone yet it, appears all this is based on an antenna perfectly isolated from ground with purely resistive radiation resistance/zero ground loss with only losses in coax.

Rarely does theory, lab, in the field, show the same results in anything,


i always assumed the transformation in a q section was down to use of an intermediate value of cable being used that lay between target and real impedance and takes advantage of the q sections abilty to invert impedance. if i recall its actually 72 ohm cable that should be used in theory using the formulas, but 75 ohm is standard,widely available and cheap as its used/manufactured in vast quantities for household use and is close enough in nominal impedance to allow an almost perfect transformation in the field as most likely other factors will play a bigger part in efficiency or inefficiency whatever the case may be.

i don't believe its possible to stop common mode current that arises from imbalance in the system design,in other words without a balanced radio,balanced wire feed and balanced antenna,the object is to minimise them with as much shield impedance as is possible, but even megaohms would still leave a tiny insignificant imbalance and a minute curent flow, even insulators will allow an avalanche current flow if you bypass the breakdown voltage and will have a minute flow below that,varying on type of insulator,i don't think a perfect insulator exists and i don't think you can completely kill common mode current on an imbalanced system,but you can minimise its effects to practically zero

well thats my tuppence worth, i'm sure many will disagree, but being wrong is part of learning and i'd rather be wrong and learn than insist i'm right when i'm not sure, and tbh i don't think many people are sure or this debate wouldn't be happening.

Jazz.

 

[bob85]

ck made a post about ground loss and how it figures into the equation jazz, i can't find it but its on here somewhere.

 

[jazzsinger]

ck made a post about ground loss and how it figures into the equation jazz, i can't find it but its on here somewhere.

My bad mate, I must have missed it :redface: , hope your well mate