VSWR doesn't matter?

Hi Jan,

Actually, there is a transformer there in the typical Ham transmitter (and probably in every general purpose power source) that typically transforms the native Z to the output Z. This is a step up for solid state, and step down for tubes. In the solid state rigs, it is a literal transformer feeding the 1-2 Ohms through a 5:1 winding ratio to a switched bank of low pass filters. This stuff is mud ordinary.

As for the reflected energy, depending upon the phase it will either combine destructively (heat) or constructively (cool) in the extremes. There are, of course, 179 degrees of variation between these extremes before they repeat themselves again. Cooling, of course, is something of a misnomer as nothing useful is happening (poor power transfer) so perhaps the terms should be destructive through uselessly benign.

73's Richard Clark, KB7QHC

In my opinion the simplest way to answer your question is that you are assuming that the transmitter is equivalent to a 50 ohm load, which is not true because the transmitter is instead equivalent to the series of a 50 ohm load and a voltage generator.

A simple DC example grossly clarifies thre issue: connect a 12V battery to the series of a 50-ohm load and another 12V battery. How much current flows through the load? Naught (assuming the correct polarity). So no power is dissipated in it.

73

Tony I0JX

As pointed out, VSWR does matter. A lot of bouncing means you heat the transmission line with the power instead of radiating the power. 'Doesn't matter', really means it can be tolerated if need be.

Here is what you are missing. In the case of the output, (real/resistive component of the transmitter), seeing the reflected wave, it is _not_ reflecting that power back up the transmission line as you think it is. It would go back to that real impedance and heat the transmitter. Here is what is done with a miss match in the real world.

trans-output -> match -> line -> antenna

The 'match' is where the magic happens. All the energy coming down the line that got reflected from the antenna 'sees' the 'trans-output -> match' as a perfect reflector and gets bounced back[*]. On the other side of the match is the trans->output. There the trans->output sees a perfect impedance, (technically, the conjugate of the trans->output), so that all the power travels through the match toward the antenna.

The magic is that when the match is tuned, both of the above conditions are satisfied.

*The reflected wave sees a purely reactive reflector not just because of the network but also because of the output power of the transmitter. Without transmitter power the impedance as seen from the load will dramatically change.

Best, Dan.

Hi Tony,

Turn the second battery over. Double the power is dissipated in it.

Phase, you can't live with it, you can't live without it.

73's Richard Clark, KB7QHC

Kewl, then I'll just run a tap directly to the 5000 ohm plates and start a long chat up ... what the heck is all those pi matching components in the way of the rf? Probably some loss there! ROFLOL!!!

JS

--
http://assemblywizard.tekcities.com

Saying that SWR doesnt matter is a rather broad statement(like saying never or always) but I have know of antenna systems having an SWR of 30:1 and his was normal. The feedline was balanced line made of 1 inch copper. Of course an SWR lie this on coax could be fatal to coax and equipment. A more common example of this is the 1/4 wl matching section on a J-pole antenna. It matches 50 ohms to a few Kohms so an SWR of 60: 1 or so would not be unusal here.S oas long as the feedline can handle the current and voltage peaks without much los it doesnt matter much as long as the source impedance is matched to the impedance at the input to the transmission line.Im sure there is a practical limit though.

Jimmie

If you want a quick lesson in high vswr find a ham with an old tube transmitter and see if he will hook it up to a mismatched load. The cherry red plates are the reflected energy being absorbed. Transistors will just turn to smoke under the same conditions.

Dave WD9BDZ

Hi david

Wouldnt it be OK to have a high VSWR along the transmission line if the "tank ckt" can be adjusted to match the load to the transmitter output impedance? That is, the VSWR along the transmission could concievely be high, yet, with proper "tank ckt" adjustment that impedance seen by the output circuit (plate) wouldnt result in a "cherry red plate". What I am asking is ? is the transmission line VSWR directly related to "plate reddening"? I'm more asking than *telling*.

Jerry

Unfortunately, you'd be learning the wrong lesson.

The cherry color is due to the transmitter being loaded with an impedance it's not designed for, causing the final to run at low efficiency. You can disconnect the antenna and replace it with a lumped RC or RL impedance of the same value and get exactly the same result. Alternatively, you can attach any combination of load and transmission line which give the same impedance, resulting in a wide variation of "reflected energy", and get exactly the same result. All that counts is the impedance seen by the transmitter, not the VSWR on the line or the "reflected power".

The problem is that the idea of "reflected energy" turning the plates hot is so easy to understand, that people aren't willing to abandon it simply because it isn't true.

See

for more.

Roy Lewallen, W7EL

Yes! All that matters to the transmitter is the impedance it sees. It doesn't know or care that you've mathematically separated the delivered power into "forward" and "reverse" components. It doesn't know or care what the SWR is on the transmission line connected to it, or even if a transmission line is connected at all.

Absolutely not.

That's the first step in learning.

Roy Lewallen, W7EL

It appears that you agree with that part of my post but you are drawing an invalid conculsion from it. I never suggested that the passive matching network usually found in a transmitter output is unnecessary.

Bob

snipped-for-privacy@manx.misty.com (Don Klipstein) wrote in news: snipped-for-privacy@manx.misty.com: ...

... I thought we put one of these fires out just a few days ago!

Owen

Jerry;

The point I was trying to make is that the reflected current is disapated as heat in the finals if the transmitter isn't matched to the load. In a tube radio the tank circuit is the equivilent of an antenna match/tuner and converts the 2000 or so ohms at the plate to the 50 ohms of the transmission line and the unknown ohms of the mis matched antenna.

Dave WD9BDZ

As Tim Williams alludes, it depends on the transmitter design. It will often be complex rarther than resistive. Since the active device changes impedance during a single cycle of the RF signal it may not even be adequately described by a single value in ohms for a paticular frequency if you wish to analyse the case of forward and reflected power.

Consider a class C or class E output stage with an output transistor that is low impedance during most of the positive half of a cycle of signal and mostly somewhere near open circuit for the negative half of the cycle. It seems to me that the effect of reflected power is going to be different depending its phase relative to the forward power. I think this also applys to a lesser extent to a class A PA with a nice hi-Q tank circuit.

As usually whan this topic comes up, It don't feel like we have arrived at a usefull and convincing model of what happens, possibly because simple descriptions don't cover everything.

Bob

Hi Bob,

No quantifiable answer I see. It's not unexpected, everyone who knows what it isn't has never been able to say what it is. It seems like the stock answer you give the cop who asks if you know the speed limit. "No. But I wasn't speeding!"

The dependency here started with a conventional Ham transmitter, one so ordinary as to be a commodity. The design is not so exotic as to elude a very simple value - except for those who know it isn't 50 Ohms.

73's Richard Clark, KB7QHC

Class D rules. (Using MOSFETs, the Thevenin equivalent is quite easy to spot, too!)

Tim

-- Deep Fryer: A very philosophical monk. Website @

"Roy Lewallen" wrote

_____________

But reflected energy/power does exist.

For an easy example, such reflections are evident in the picture seen on an analog TV receiver when the match between the transmit antenna and the transmission connected to it is bad enough.

In analog TV transmit systems with a typical 500+ foot length transmission line from the tx to the antenna, a 5% reflection from a far-end mismatch can be quite visible, showing as a "ghost" image that is offset from the main image as related to the round-trip propagation time of the transmission line.

RF

Hi Jimmie, Keep in mind I'm answering in the context of the op's post. And the theoretical SWR on a stub is infinite. The point of the stub at the antenna is to keep the SWR on the transmission line in a reasonable range, to make a match if you will. To put high SWR on the feedline instead of matching at the antenna isn't a great idea in my book.

OTOH. I finally did some sidebanding a couple of months ago. (First time on HF.) I got my hands on an old swan 500c. After changing the 6je6's and supply caps, I had to find out what it was like to get on the air. I ran outside and hung a wire between the lab and the shop. 40-50 feet. Put a couple of alligator clips on the end of a chunk of rg-58 and into the window. I started looking for the antenna through the trans-match with an antenna bridge. The tuning was very sharp, lots of Q. I don't know if I could have found it without the bridge :) I was willing to tolerate the miss match to get on the air.

Well, it worked out. I made some great QSLs across the mid west and into northern CA. I live in Vernon AZ. I'm pleased this turned out to be as great a radio location as I thought. It shouldn't be long before I get a beam on a tower. By then I'll look to match at the antenna and keep the SWR off the feed line as much as possible.

Best, Dan.

Well, without a line, you don't have a real component to tune into. Drawing arcs on a smith chart from an open line with capacitors and coils will only get you to another purely reactive point.

Best, Dan.