Of SWR and Tuners

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Steve - WB3HUZ:
With all the talk last night of SWR and tuners, especially with some station's antennas icing up, and since Walt, W2DU has recently joined this forum, I thought it would be a good time to delve into this subject. In short, especially on the lower bands and with relatively short runs of coax and/or low-loss coax, SWR is generally not a big concern.

If you are running a rig that requires you to tune and load the final, just tune and load the final. Don't worry about the SWR. If you can tune and load your rig to the proper plate current value and no arcing occurs in the final tank, you are probably good to go - NO TUNER needed. Make an old buzzard transmission. If you notice little or no heating in the coax, you are most certainly good to go. Don't believe me? Read this chapter below from Walt's "Reflections II" book, especially section 7.5

http://www.w2du.com/Chapter07.pdf


I recommend you read the entire book, since it will put everything in context and make clear the basics.


My 80/75 meter antenna is a dipole, resonant around 3820 and fed with 100 feet of RG-8 coax. I have used it without any problems over the range of 3700 to 3900 kHz. I'm sure I could cover a larger frequency range, if desired.

Using GNEC to simulate the antenna and W9CFs transmission line applet to look at the feedline, I checked things at 3700 and 3900 kHz. Here's what I found.

 - 3700 kHz

Dipole feedpoint impedance: 80.1 - j54.042 (SWR: 2.57:1)
Impedance at the end of 100 feet of RG-8: 36.59 - j35.65 (SWR: 2.33:1)  * SWR is slightly lower than at the feedpoint due to loss in coax.
Loss in the coax: 0.5  dB   * Matched loss (with no SWR) is 0.36 dB, so only an additional 0.14 dB of loss is incurred due to the SWR.

 - 3900 kHz

Dipole feedpoint impedance: 94.183 + j34.495 (SWR: 2.22:1)
Impedance at the end of 100 feet of RG-8: 67.21 - j40.39 (SWR: 2.07:1)  * SWR is slightly lower than at the feedpoint due to loss in coax.
Loss in the coax: 0.44  dB   * Matched loss (with no SWR) is 0.37 dB, so only an additional 0.07 dB of loss is incurred due to the SWR.


Notice the additional loss in the feedline due to the SWR in both cases is a tenth of a dB or less! That would never be noticed on the air and will not cause heating of the coax. Putting a tuner in line would only produce additional loss.

flintstone mop:
OK FB Steve,
Thanks
It's worth a try to tune and load the transmitter/linear and let 'er rip. It saves that extra step of tuning the tuner.For Ham use, there should be no danger of coax heating or major crap outs. Stations running a few thousand watts or 50KW definitely have to worry about excessive SWR.
I feel for the FM stations on a budget that do not have radoms or heaters on their ants with the freeze and ice we are experiencing now. We always lowered the RF out for those few critical hours to protect the feedline/antenna/transmitter
Fred

K9ACT:
I was going to post a new topic but the subject here is about what I would have called it, although it address different issues.

There is an article in World Radio about the evils of the new tests for hams and specifically about Extra Class.

The gripe was that there was nothing in the exam about the importance of where the wattmeter/SWRmeter is located in the transmission line, i.e. before or after the tuner.

As the commercial Swr/tuners I am aware of have "antenna" terminals, one would be lead to believe that the tuner is always just preceding the antenna.  In my Heathkit tuner, the antenna is connected to its output also so the meter/bridge would always be on the transmitter side.

Am I missing something?

js

KF1Z:


The meter would naturally need to be between the transmitter and the 'tuner'... of course...



Not sure why anyone would complain that it isn't a test question though...

If it did need to be on a test, I would put it in the Technician pool, not the Extra!

KA8WTK:
I don't think you are missing anything....

Xmitter ------> SWR Bridge ---------> Tuna -----------> Antenna

At least, this is what you might need to do with a new type solid state xcvr.

What Steve is saying is that with a tube final, that will properly dip and load into the antenna, there is no need for the tuner and the coax losses from the SWR don't amount to much. If it dips and loads, use it!
However, if you get any arcing or heating, you may need to resort to the tuna and SWR bridge.

Heck, half of these new rigs have auto-tuners built in. The new-fangled appliance operator doesn't know how to adjust a tube final or a tuner. He just pushes a button.

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