It works if you're concerned about mismatch loss. However, if you just plug the 50 ohm coax cable into the 75 ohm antenna I think you can tolerate the 1.5:1 VSWR and 0.4dB mismatch loss.
Voltage reflection at the 75 ohm end is: T = (50-75)/(50+75) = 0.2 Voltage into your 75 ohm input is: T = 1 - (0.2^2) = 0.96 Converting to dB, you will burn: Ploss = 20 Log(0.96) = 0.35 dB in mismatch loss:
Much also depends on the frequency range. Tolerating the mismatch can work over a wide range of frequencies. 1/4 or 1/12 wavelength transmission line xformers only work around the cut frequency (and its harmonics).
Most of the antennas on my roof are allegedly 50 ohms. However, I'm cheap and use mostly cheap CATV coax (RG-6/u) with Type F to BNC adapters. No equipment fatalities or anomalies experienced. As an added bonus, the loss in the 75 ohm coax is slightly less than the loss in the equivalent size 50 ohm coax, thus compensating for the tiny mismatch loss.
If I'm doing measurements and the 1.5:1 mismatch causes excessive errors, I use a 50-75 ohm minimum loss pad: I'm cheap, so I make my own: I have a pile on the workbench for using CATV hardware for 50 ohms devices and test equipment.
OT stands for Out of Time. Gotta run.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
On a sunny day (Sun, 07 Sep 2014 10:00:19 -0700) it happened Jeff Liebermann wrote in :
Yes, right. I found an old 4 x bowty in a cabinet from the analog TV times. Just realized that channel 14 (US) or 21 (Europe) is around 430 MHz. And the thing still has 9 dB gain at 70 cm. Added 20 meters RG58.. put it in the attic, and so far it worked great. But the missmatch was bothering me.
Yes, anything not close to 1.2 or less bothers me...
Take it easy... I have done the stress running with 3 hours sleep on everage and a lot of travelling last week, and setting myself up for an other run, but this week no.
Pull out the RG58 and run RG6, stay at 75 ohms until you get to the radio. You will gain by using the lower loss RG6 and it will easily make up for the one mismatch going from 50 to 75 ohms.
Ok, I agree with Jeff that you should just not worry about it. Your losses above and beyond your normal cable losses are minute and not worth the effort to correct. Your losses will be much greater when perfectly matched than when mismatched with 1.5:1 impedance mismatch.
If you are using at least some decently high frequency, why not use a microstrip construction on a PCB ? With a tapered transmission line, you might do additional tricks.
On a sunny day (Sun, 07 Sep 2014 20:50:56 +0300) it happened snipped-for-privacy@downunder.com wrote in :
Yes that is the neat way of doing it, but this is a one time experiment, making a PCB is more work than cutting two 8 cm cable pieces and soldering those together. We live in a peeseebee infected world really. Peeseebees require chemicals dangerous to the environment. I think we should really propose to the EU that all peeseebees should be eeeheh outlawed? reduced by 50 %? With a bit of luck some poly-tician is reading this. think how it will put you in the spotlight!
All PCBs are not made with "dangerous" chemicals. You can use milling techniques or an equivalent can be made up using copper strips which might be very easy in the case of your circuit.
They have to be inconveniently long to work well, though.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
Well, perfect balance is just what we'd all expect from you, Jan. ;)
I didn't know the lambda-over-12 trick, but it's perfectly reasonable in retrospect. You can make the reflections sum to zero by using two equal-strength discontinuities 180 degrees apart, or three of them 120 degrees apart, or four 90 degrees apart, and so on. The fact that you can make the three equal-strength discontinuities using two sections of the same lines you're trying to match is cute. Thanks for pointing it out.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
Sorry for not being overly pedantic: "Mismatch loss in transmission line theory is the amount of power expressed in decibels that will not be available on the output due to impedance mismatches and signal reflections".
Instead of "mismatch loss", would the "loss (in dB) due solely to the reflected power that is returned to the source" be a more useful term? In other words, just the loss due to the mismatch, not the loss for the entire cable (including connector loss and shield leakage). In order to obtain the end to end cable loss, which I believe is what you're pointing out, one simply adds the coax loss, mismatch loss, connector loss, shield leakage, internal reflection losses due to excessively sharp bends, dissimilar metals resistive connector loss, and whatever else I forgot.
Incidentally, there's a similar superstition that the "impedance bump" caused by some coax connectors contributes to horrific losses leading to the assertion that coax connectors should be avoided. Baloney: Note the large number of PL259/SO239 connector pairs, which are certainly nowhere near 50 ohms. At 2.4GHz, the end to end loss is
3.2dB. At 450 MHz, it's about 0.16dB. I've also done this test with BNC "T" connectors, and much longer strings of adapters, with similar low losses and decent VSWR sweeps.
Unless you're talking to space craft and satellites, don't worry too much about mismatch loss.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Get used to high VSWR antennas. They're everywhere. For example, there's the Icom AH7000 and Diamond D-130 discone antenna model: It's advertised as a "broadband" and "50 ohm" antenna. I think it might be 50 ohms at about 260 MHz but nowhere else: Plenty more examples if you need them.
Actually, a good example of a broadband antenna that's close to 75 ohms are the various commercial VHF/UHF TV antennas. You can find the Eznec files at: Grind out any impedance versus frequency plot and see how "flat" the VSWR curve appears. I can assure you that it won't be >OT stands for Out of Time. Gotta run.
Ok, I'm sympathetic. I spent all of yesterday in bed doing absolutely nothing. Today, I pay the price of my laxitude by preparing the house for winter and doing a service call (reinstall a candelabra at a customers). The RF bench currently has a weed whacker rebuild in progress. No RF until it's done.
I'm officially retired and too busy to play.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
I did not remember the 1/12 figure specifically, but I knew that given a couple of hunks of cable of sufficiently different impedances you can match to just about anything. As with RC circuits, the bigger the impedance difference you're trying to match, the lower the maximum bandwidth you can achieve with the match.
"Mismatch loss" was clear enough, made even clearer by the context.
More troubling is the unreliability of common connectors (usually "F"S). In analog TV, or even FM radio, reflections from crappy connectors can cause a lot of unnecessary trouble. Either way, the fewer connectors, the better.
Loss, not too much, OTOH reflections can really screw things up.
I like F connectors. If you count the connectors that are in use world wide, the number of F connectors far exceeds all the other connectors. Civilization would come to an abrupt end if F connectors were as bad as you suggest.
I make it a point of using as many F connectors as possible, just to see how well (or badly) they work (and because I'm cheap). I can't say that I haven't had problems, but in general, they're "good enough" for consumer use. I would not want to send one into orbit or use one in a critical application. There are better connectors. Oddly, the effects of reflections have not been a problem for me.
Some of the problems I've seen with F connectors:
Push-on connectors work. Crimp ring connectors suck.
Mixing alodine 1200 plated aluminum connectors, stainless connectors, nickel plated connectors, tin plated brass connectors, tin-silver-zinc alloy plated connectors, gold over nickel-phosphor plated connectors, and the unidentified plating connectors coming from China are invitations to problems. Besides electrolysis problems, the ferrous materials produce PIM[1] problems.
The RG-6/u unplated copper center conductor and the tin plated center contact on the F connector receptacle, are a marginal proposition.
Let me reflect on that.
Here's the SCTE F connector test for reflections and return loss: Looks like they changed the test method. Oh-oh. More: I'm not sure, but it looks like they want to see -30dB return loss or better. I typically see -40dB to -50dB or more. No problem.
[1] PIM = Passive InterModulation Distortion:
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
On a sunny day (Sun, 07 Sep 2014 13:22:53 -0700) it happened Jeff Liebermann wrote in :
Yes, that 4 bowtie comes out about the same as mine.
formatting link
Yes I've been officially retired for some years, and become more and more active it seems. All the things I never had time for anyways I 'need' to do now. Challenges?
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