Sounds logical. Agree or disagree?

[Captain Kilowatt]

Here's a little tidbit you won't find written anywhere else to the best of my knowledge.

Coaxial cables having effective lengths longer or shorter than ¼
and ½ wavelengths and their multiples can be used as impedance transformers, and
provide a means for matching elements of impedance mis-matched system devices
together. Cables longer than ½ wavelength exhibit an inductive effect and those shorter
than ½ wave become capacitive.
Therefore, the trick in matching mis-matched systems together is to find a cable
length that produces equal and opposite inductive or capacitive effects to arrive at a net
resistive match. The means of accomplishing this are the most important tools that cable
network designers have to work with.

That's a bit presumptuous don't you think? Guess you never heard of the conjugate match. Been known about and written about on the internet for decades.Even before that in what were called books. I needed a piece of 35 ohm transmission line to match my 50 ohm transmission line to a pair of 13B2 yagis, each of 50 ohms that when feed in parallel resulted in 25 ohms. The required matching system has to be between the two impedances to be matched. In this case it had to be between 25 and 50 ohms. As it happens a 1/4 wavelength of 36 ohm coax will effect this match. I made a 1/4 wavelength section from square aluminum tubing and 1/2 inch copper pipe, the sizes of which created a 36 ohm line section. Two pieces of 75 ohm cable in parallel would do the same thing.In the end I ended up with two 50 ohm antenna in parallel making 25 ohms that when connected thru a piece of 36 ohm transmission line resulted in 50 ohms at the transmitter. Anyone that is familiar with transmission lines knows all about impedance transformation using coaxial line sections.

 

[RatsoW8]

When I was a new ham living in VA with no money I made a 2 meter antenna from a base loaded mag mount CB antenna I bought at a yard sale for $2. Re-tapped the coil and measured swr with a free (from my Elmer) swr meter that just happened to be fairly accurate on VHF freqs. Installed in my Ford Ranger with a silly randon length of coax and a 10 watt Heathkit rock bound 2 meter rig. I could be at Cape Point in Buxton, NC and hit repeaters in Raleigh. I did that without the internet or consulting with an engineer. Radio use to be amazingly fun and easy.

I stil apply the "whatever" coax length theory to all my antennas and I'm still working the world.

 

[bob85]

The article is not such a good read, he says

"When the line and the devices to which it is connected
exhibit
identical
impedances, signal radiation from the line’s outer conductor will then be
at a minimum.
Be aware, however, that when mis-matc
hes are present, “standing waves” can
appear along the outer conductor that will
radiate signal power"

vswr does not appear on the outside of the shield causing radiation,

if you terminate a piece of 50ohm coax with a resistor other than 50ohm you get reflections & your vswr meter swings but no radiation,

common mode current on the outside of the braid causes radiation not vswr,

most people building amps especially tube amps from scratch know that input & output impedance & optimum load impedance & how much current your bias supply must provide changes with drive level, class of operation & supply voltage,

The ones that don't sell davemade style keydown boxes to people on stock volts giving shitty efficiency and talk bollocks on forums about the amps output impedance not been 50ohm causing high reflect.

 

[Tallman]

. Anyone that is familiar with transmission lines knows all about impedance transformation using coaxial line sections.

Such as a 6' section of 75 ohm coaxial stub match? Attached to the antenna and shorted on the other end?

 

[Captain Kilowatt]

Such as a 6' section of 75 ohm coaxial stub match? Attached to the antenna and shorted on the other end?

Sounds like a 1/4 wave stub for reducing the second harmonic of 27 MHz or thereabouts if the cable has a Vf of about 66%. It looks like an open stub at 27 MHz but it also looks like a dead short at 54 MHz. My Mirage 2m amp has a short piece of small Teflon cable across the output jack. It acts as a harmonic filter to keep the 144 MHz signal out of the 288 MHz range especially since military air comms are in that range.

 

[N0NB]

I stopped reading when the phrase, "fool the transmitter", was used. Once someone utters that phrase, their technical credibility is shot with me.

 

[rabbiporkchop]

I stopped reading when the phrase, "fool the transmitter", was used. Once someone utters that phrase, their technical credibility is shot with me.

You took it out of context. The writer of the article was referring to unscrupulous cb shops practices.


""This method was popular many years ago in the CB business when someone used the wattmeter in the reflected mode and began cutting the coax cable from the antenna until the meter read zero. This fooled the transmitter, but the cable mismatch was still there.

"Ya got yer coax cut" was the catch phrase of the time."""

Notice the condescending tone. He's referring to other people's practices not his own practices.

 

[Captain Kilowatt]

"Ya got yer coax cut" was the catch phrase of the time."""

Back when I was on 11m, about a hundred years, ago that phrase either meant that you couldn't hear or get out worth a damn or that you literally had your coax cut by someone.

 

[Road Squawker]

You took it out of context....

""This method was popular many years ago.... This fooled the transmitter, but the cable mismatch was still there...
Notice the condescending tone. He's referring to other people's practices not his own practices.

Actually, you might want to re read it, He clearly states that "THIS FOOLED THE TRANSMITTER".........

it incorrect statement has nothing to do with "tone"

 

[Captain Kilowatt]

Actually, you might want to re read it, He clearly states that "THIS FOOLED THE TRANSMITTER".........

it incorrect statement has nothing to do with "tone"

Pssssssst.....it's one of those 11 meterisms. I usually just roll my eyes say "Uh-hunh" and nod.

 

[Road Squawker]

yeah, I can accept it when someone says something like "it makes the transmitter "happy",........

that tells me that they know the transmitter wants to "see" about 50 ohms, and that the tuner is providing it.

.

 

[bob85]

The writer is also claiming that trimming your cable for a lower reflect using a watt meter in reverse ( vswr meter with a different scale ) is only fooling the transmitter,

i would say if the vswr/reflect changes significantly with small changes in coax length you have cmc on the outside of the coax shield,
trimming the coax is changing the common mode impedance seen in parallel with the load,
vswr is changing with coax length not fooling the transmitter and its not due to transformer action of the coax which only effects impedance seen at the transmitter,

the impedance the transmitter sees will swing around whenever the load is not matched to the coax but vswr can't be changed significantly by trimming unless you have common mode issues.


ATU's fool the fools who believe an ATU only fools the transmitter.

 

[fourstringburn]

ATU's fool the fools who believe an ATU only fools the transmitter.

Well it sure doesn't tune the antenna if that's what you are getting at.

About the only time an antenna tuner can tune the antenna sort of speak is when it's a remote antenna tuner mounted right at or very close to the antenna feed point.

 

[bob85]

The idea that ATU's just just fool the transmitter by making it happy with the load its seeing allowing full TX power but do nothing for the antenna system is a myth,

ATU's don't reduce the vswr on the coax but they do when adjusted for a conjugate match at the transmitter end of the coax bring the system to resonance,

The reflected wave from the mismatched coax-antenna junction is re-reflected at the ATU's output & added to the incident wave giving reflection gain at the ATU
( more power than your transmitter outputs ) reflection gain cancels the reflection loss at the antenna,
with low loss coax and quality tuner almost all of the transmitters output is delivered to the antenna.

That's what I'm getting at, its been a topic on here several times. ATU's fool the fools who believe an ATU only fools the transmitter.

 

[N0NB]

@rabbiporkchop

He wasn't quoting anyone with that line. He was using the vernacular that became prominent sometime in the late 20th Century. I've never understood how one can "fool" an inanimate object. Does putting fuel system cleaner into the tank "fool" the engine into running better? No, it does a very specific thing that causes the engine to run better.

Modern solid state transmitters are designed with protection circuitry that reduces the output power in the presences of a high impedance mismatch. The engineers that designed such circuits understood the physics of impedance mismatches and sought to minimize the damage to the equipment should an antenna system failure occur.

One of the first things one learns in electronics school when being introduced to AC theory is that maximum power transfer occurs when the impedance of the source and the impedance of the load are equal. Impedance matching between stages is designed in and cannot be twiddled with. Transmitter finals rarely, if ever, have an internal impedance of 50 ohms. In the case of solid state transmitters broadbanded output transformers are used to transform that impedance to 50 ohms. So does the external matching network mislabeled as an "antenna tuner". Even AM broadcast stations use a matching network between the antenna tower/array and no broadcast engineer will say that the transmitter is being fooled. This term is something that entered the amateur radio vernacular in the late 20th Century and belies an incomplete understanding of the concepts involved.

@bob85

You got it right.

@fourstringburn

While "antenna tuner" is a misnomer, there is no doubt that it provides a conjugate match of the antenna system to the transceiver. Johnson probably had it right by calling theirs a Matchbox.

Even though I know the theory well, it occasionally catches me off-guard when I've used the tuner in the K3 and I see the SWR reading on the P3 showing 1.9:1--happened to me yesterday on the Elecraft Net, in fact. This, after reading Walt Maxwell's Reflections nearly 25 years ago.