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.
The fact that any transmission line and antenna combination can be replaced with an RLC lumped load at the transmitter output and the transmitter can't tell the difference is something that a lot of hams seem to have a problem understanding.
What I've never understood is why so many hams have a problem with the concept of equivalent circuits only when antennas and transmission lines are involved.
-- Jim Pennino Remove .spam.sux to reply.
Sure yoiu can, that stub is a transmission line. It would matter if it s a
1/4 wl long or 21 1/4 wl long. If it is designed to handle the current and voltage peaks it can transmit power with low loss when a high VSWR is present. Its just that most people dont make there feedlines out of inch copper tubing. Even with 450 ohm ladder line 10:1 VSWR is very acceptable.
I'd have to say that as soon as a circuit contains a radiator or a transmission line the arm waving begins.
-- Jim Pennino Remove .spam.sux to reply.
It also isn't true that there is no energy in the reflected wave, that such beliefs are gobbledegook, and that RF standing wave energy just sloshes around in a transmission line at less than light speed. To really understand what is going on, one has to understand superposition and interference between RF energy waves. You are on record as not caring to understand reflected energy. Please don't condemn those of us who are trying to understand.
--
73, Cecil, w5dxp.com
Think about this - if the transmission line is exactly one-wavelength long and lossless, the transmitter sees exactly the same impedance as the load. At the load, we know reflections occur, but they are same-cycle reflections so during steady-state with no modulation, exactly the same conditions exist at the transmitter as exist at the load if the transmitter has the same impedance as the transmission line. So even if we cannot measure the reflections back into the transmitter, they are no doubt, there - that is, unless one denies the existence of reflections in which case, one needs to explain how standing waves are possible without reflections in a single-source system.
--
73, Cecil, w5dxp.com
Hi Jim,
Would it be fair to say there are a number of Hams (no need to go into proportionality, could be equal number) who have difficulties of understanding with going from lumped, equivalent circuits to antennas and transmission lines?
The two perspectives are not exclusionary nor mutually incompatible, only the arguers are.
73's Richard Clark, KB7QHC
Amen
In that case...
Half power is only lost when terminated to a 50-ohm load; i.e. no standing waves. What happens when there's a mismatch and reflected energy :)
I'll go stand in a corner...
A mismatch where, between the feedline and the antenna, feedline and source, source and impedance seen at the input to the feedline.
Jimmie
can
I've noticed that, at least in this area, Fox 39 (WQRF) has a ghost of a few microseconds (I forget what exactly, I've calculated it before). Something like 500 feet, IIRC.
Tim
-- Deep Fryer: A very philosophical monk. Website @
I've been reading the posts on this. One poster said this has been going on for twenty years! (For the other groups, this thread has life on rec.radio.amateur.antenna) It doesn't need to be so.
First, there should be no doubt that reflected power on a transmission line is real. Sure, you can replace the line with a lump but that doesn't clear up the question for others.
For the next two examples, see page 179:
First example, step into an open line with a Thevenin source. The energy is divided between the source and the line. Half the energy is moving down the line and when it returns changes the impedance the source sees to an open circuit. The energy does not flow back into the source, so, where did it go? It is stored in the capacitance of the line.
Second example, step into a shorted line. When the energy returns the source now sees a short. The energy does not flow back into the source, so, where did it go? It is stored in the inductance of the line.
So here are two examples where the energy sent down the line do not return to the source.
Third example. Send a pulse down the line. The Thevenin voltage source will go to short, as it should, when the pulse falls. The pulse is reflected from either an open or a short at the end of the line. All the energy is dissipated in the source impedance when this pulse returns. That is where the energy goes. And it is obviously the _same_ energy created at the source.
Sure, non of the cases above represent steady state AC. But they do show that energy may or may not be returned to the real component of the source.
With the above in mind, it can be shown, (in some part II), that a real accounting of energy from source to load and back is possible. Equivalent circuits are just that, the trading of line for lump. But, and this is really important, the only reason the effective impedance at the input of a 50 ohm line is not 50 ohms is because of reflected energy.
Best, Dan.
Do us a favor, compute the S-vectors for an incandescent lamp with a linear filament. Then follow though with the same for a transmitter, transmission line and a mismatched load. You will find that is the reflected S-vector that adds heat to the plate.
-- JosephKK Gegen dummheit kampfen die Gotter Selbst, vergebens. --Schiller
Poppycock, TV ghosting is caused by multipath length differences. Calculate the position ratio and the horizontal scan frequency (15750 Hz is close enough). That gives you the path length difference; it is generally on the order of miles (= major terrain features).
-- JosephKK Gegen dummheit kampfen die Gotter Selbst, vergebens. --Schiller
Except for less than 1%. In the 1960's that would have been about 5% to
10%, and handwaving will never include Wes Hayward W7ZOI.-- JosephKK Gegen dummheit kampfen die Gotter Selbst, vergebens. --Schiller
Wrong, wrong, wrong. Energy is not created or destroyed, but it can be converted back and forth to mass. See mass defect and hydrogen fusion.
Less wrong, line length makes no significant difference (unless it is really lossy).
Yes they do. Same model, same reflected power, same heating effect on the last amplifier stage.
Wrong, wrong, wrong again. Transmission line impedance is strictly a matter of physical dimensions, and surrounding materials permitivity and permeablity.
-- JosephKK Gegen dummheit kampfen die Gotter Selbst, vergebens. --Schiller
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