[Repeater-Builder] Cable formula
It looks like I am able to post here again. Don't know what happened. This subject was examined a little about a month ago. You can search for my comments back then but I will state again similar to what I said then. Some transmitters do not like to see any reactive load on them. They will cut back the power out and/or generate some spurs in the presence of a reactive load. Hooking a transmitter to a dummy load or an antenna with a pretty flat swr can present a good load to a transmitter as the load is broad band and so are most antennas fairly broad band. But when you put a cavity/duplexer on to a transmitter it is anything but broad band. It can have a good match/return loss/swr at the wanted frequency but at the same time be very reactive off frequency and the particular transmitter may not like that reactance. It is not just a matter of tuning the duplexer to present a good return loss at the wanted frequency as is done with a network analyzer or other method of tuning. The transmitter may still not like it for the reasons stated above. As Jeff has stated, the transmitter is not a 50 ohm source but is set up to deliver power into a 50 ohm load. Its output impedance may be far from 50 ohms but it doesn't matter as that won't affect the tuning of the duplexer. As long as the transmitter can deliver power to a 50 ohm load we really don't care what its output impedance is. However what impedance is presented to the transmitter as a load is important and that depends on the cavity tuning and whatever the interconnect cables does to the impedance if the cavity is not presenting a pure 50 ohms. The cable will transform that impedance to something else. This can be on frequency reactance and/or off frequency reactance. There are several ways to correct the problem of a transmitter not liking the load that a duplexer presents. The best and most expensive is to use an isolator at the transmitter. This always presents a flat 50 ohms to the transmitter. It may not transfer the most power if there is some (on frequency) mismatch at the duplexer though. Some of the power as the result of a mismatch at the cavity will go to the load on the isolator and be lost. Then there is the small inherent loss of the isolator and low pass filter too. As a side note here; an isolator can rob you of power into the antenna if the antenna/and or duplexer have reflected power. All the reflected power presented to an isolator goes to the isolator load and is lost in heat. Without an isolator nearly all the reflected power that is present will make it back to the antenna and be radiated as it gets re-reflected when it reaches the transmitter. That is of course if the transmitter is happy working into a reactive load. The other ways to correct the problem, other than using a different transmitter that is not bothered by reactive loads as much, is to use a Z match or try different length cables that make the transmitter happier. Both of these transform the impedance/reactance presented by the duplexer to something more palatable to the transmitter and allow it to produce the power intended. There is no sin involved in using different length cables to make the system work properly. It is not a band-aid approach to mask other problems. The real problem is that some transmitters, because of the way they are designed, do not like reactive loads. Some of the duplexer manufacturers tell you about trying different cable lengths in their tech notes for these very reasons. Close spaced duplexers will be the worst with off frequency reactance as the impedance has to change quickly as you move away from the wanted frequency in order for the duplexer to do its job. 73 Gary K4FMX
Re: [Repeater-Builder] test
You are full quieting. Chuck WB2EDV - Original Message - From: "Gary Schafer" <[EMAIL PROTECTED]> To: Sent: Saturday, June 30, 2007 8:23 PM Subject: [Repeater-Builder] test > > > > > > > > > Yahoo! Groups Links > > > >
[Repeater-Builder] test
Re: [Repeater-Builder] Cable formula
I was thinking the same thing. I was wondering how the connection at the transmitter looked. My bet would be that this is where the real problem is. Chuck WB2EDV - Original Message - From: "Jeff DePolo" <[EMAIL PROTECTED]> To: Sent: Saturday, June 30, 2007 6:22 PM Subject: RE: [Repeater-Builder] Cable formula >> Do you have any thoughts on why or how a well designed Z >> match could produce cable >> radiation? > > For the feedline to radiate, there would have to be currents flowing on > the > shield. It would seem to me the only way to get that to happen would be > if > there was an imperfect shield connection at the mating connector at one > end > or the other. Most Z-matchers I've dissected use a length of wire as an > L, > with shunt trimmer caps. Assuming the enclosure itself is bonded well to > the connectors at either end, I can't come up with a good reason why such > a > device inserted in a transmission line would cause shield currents to flow > absent a connector issue. > > --- Jeff
RE: [Repeater-Builder] Cable formula
> Can't argue with your analysis. My only point is that if you > are intent on dealing with a > TX to duplexer mismatch, a half wave cable replicates what > ever mismatch exists. A > random length cable can mask the real world condition by > making the apparent mismatch > better or worse than it really is. Depends on how you're defining "mismatch". If you mean VSWR, then it doesn't matter what cable length you use, it doesn't change the VSWR. The Z at the source end will change, but who knows whether or not that's a good thing or a bad thing, so where's the benefit of replicating the load Z at the source end? > Do you have any thoughts on why or how a well designed Z > match could produce cable > radiation? For the feedline to radiate, there would have to be currents flowing on the shield. It would seem to me the only way to get that to happen would be if there was an imperfect shield connection at the mating connector at one end or the other. Most Z-matchers I've dissected use a length of wire as an L, with shunt trimmer caps. Assuming the enclosure itself is bonded well to the connectors at either end, I can't come up with a good reason why such a device inserted in a transmission line would cause shield currents to flow absent a connector issue. --- Jeff
RE: [Repeater-Builder] Lunar & Angle Linear
Skip: Most of this is pure speculation on my part. Lunar electronics was run by WB6NMT Louis Anciaux when he was in Calif. I always heard he was in the military and Lunar was a side business that he closed when he transferred elsewhere. My speculation is that Louis and Chip had a deal where Chip built the preamps that Louis sold under the Lunar Electronics label. I agree, I also liked the Jannel preamps. In the wsvhf and wsuhf world the only other person I know that is close to Chip's reputation for building preamps is WD5AGO. Tommy sold his products through Down East Microwave. John Lock kf0m at arrl.net > -Original Message- > From: Repeater-Builder@yahoogroups.com > [mailto:[EMAIL PROTECTED] Behalf Of skipp025 > Sent: Friday, June 29, 2007 11:09 AM > To: Repeater-Builder@yahoogroups.com > Subject: [Repeater-Builder] Lunar & Angle Linear > > > My impression is/was Lunar was a commercial/amateur product line with > multiple persons running the business. Lunar also made some rf power > amplifiers. > > Angle Linear is probably just Chip doing what he seems to do very > well... low noise rf preamplifiers. > > The interesting part of some Lunar Preamplifiers was the 1/4 or 1/2 > wave line at the circuit front-end. > > In addition to the Gasfet preamp brands I mentioned Bob also talked > about Janel and Hamtronics. I really, really like the Janel stuff... > but as with most good brands they were swallowed up by someone and > quickly went away. :-( > > Past a min technical design, construction and alignment preamps can > be a pretty straight-forward desk top project for most people. Since > they can be so easily made... even low cost project kits like the > Hamtronics and the pre-built Dick Smith Electronics (rip in USA) units > are pretty darn good performers. > > I'm not sure how long the Lunar brand is/was around... but I've seen > their ads in a number of old Ham Mags... Maybe someone can get Chip > to tell the story some day. > > cheers, > s. >
Re: [Repeater-Builder] Cable formula
Can't argue with your analysis. My only point is that if you are intent on dealing with a TX to duplexer mismatch, a half wave cable replicates what ever mismatch exists. A random length cable can mask the real world condition by making the apparent mismatch better or worse than it really is. Do you have any thoughts on why or how a well designed Z match could produce cable radiation? In a message dated 6/30/2007 1:03:51 P.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: What I was getting at was that the rule-of-thumb you recommended, i.e. sticking with a half-wave cable, doesn't give you any better or any worse of a chance in getting the "right" match. The rule could just as well be "whatever cable is long enough to get from the transmitter to the duplexer" and it would have just as good of a chance in making the PA happy. ** See what's free at http://www.aol.com.
RE: [Repeater-Builder] Cable formula
> Jeff > > You make some excellent points. Thanks! > > Bruce K7IJ Yer welcome, and thank you for bringing up some issues for discussion that frequently get confused by even many seasoned techs. --- Jeff
Re: [Repeater-Builder] The real secret cable formula at the link below -
This may be helpful: http://www.txrx.com/products/pdf/Duplexer_Problems_and_Remedies.pdf Chuck WB2EDV - Original Message - From: "Steve S. Bosshard (NU5D)" <[EMAIL PROTECTED]> To: Sent: Saturday, June 30, 2007 3:45 PM Subject: [Repeater-Builder] The real secret cable formula at the link below - > > http://www.antennex.com/preview/vswr.htm > > -- > Ham Radio Spoken Here !!! > EM11ma - South Mountain, Texas >
RE: [Repeater-Builder] Cable formula
> I don't think I ever suggested otherwise. I never said that > using a half wave cable would improve > anything. What I did say was that a half wave cable would > repeat the prevailing condition neither > making it better or worse and I further said that using any > variation from a half wave cable could > either mitigate the mismatch or aggravate it. Having said > that, I still think that whatever measures > you want to undertake to improve matching, utilizing a > half-wave cable is the most coherent way > to start. What I was getting at was that the rule-of-thumb you recommended, i.e. sticking with a half-wave cable, doesn't give you any better or any worse of a chance in getting the "right" match. The rule could just as well be "whatever cable is long enough to get from the transmitter to the duplexer" and it would have just as good of a chance in making the PA happy. --- Jeff
[Repeater-Builder] The real secret cable formula at the link below -
http://www.antennex.com/preview/vswr.htm -- Ham Radio Spoken Here !!! EM11ma - South Mountain, Texas begin:vcard fn:Steve Bosshard n:Bosshard;Steve adr:;;503 B. South 25th. Street;Temple;Texas;76504;USA email;internet:[EMAIL PROTECTED] tel;work:254-773-1102 tel;fax:254-773-1174 tel;home:254-770-0111 tel;cell:254-624-4230 url:http://www.bosshardradio.com version:2.1 end:vcard
Re: [Repeater-Builder] Cable formula
Jeff You make some excellent points. Thanks! Bruce K7IJ ** See what's free at http://www.aol.com.
Re: [Repeater-Builder] Cable formula
I don't think I ever suggested otherwise. I never said that using a half wave cable would improve anything. What I did say was that a half wave cable would repeat the prevailing condition neither making it better or worse and I further said that using any variation from a half wave cable could either mitigate the mismatch or aggravate it. Having said that, I still think that whatever measures you want to undertake to improve matching, utilizing a half-wave cable is the most coherent way to start. In a message dated 6/30/2007 8:54:45 A.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: Point being, if there is a mismatch, using a half wave cable does nothing to improve your chances of making your PA happy any more than would a quarter wave cable or any other random length. Without knowing the actual impedences involved, your odds of making an improvement using an X-length cable (pick your favorite value for X) are 50/50, nothing more, nothing less. ** See what's free at http://www.aol.com.
Re: [Repeater-Builder] Cable formula
Do you recall if the leakage you observed was on channel or whether it was broadband noise? In a message dated 6/30/2007 3:53:42 A.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: The cable leakage stopped when the z matcher was removed and the cable length was altered for optimum. ** See what's free at http://www.aol.com.
Re: [Repeater-Builder] Line Stretcher
Line stretchers/shrinkers were also built by Kings Connectors (I have five of them) but I don't see them in their catalog any longer. They modified a female N barrel to mate with the shoulder of a male UG-21 that permits the barrel to thread in and out of the UG-21, effectively varying the length of the device. There is a total range of about 1/2 inch, so if you center the stretcher before inserting it in the line, all you get is about 1/4 inch of range either way. That range applied to an interconnect at 1.2 gig is useful to correct an impedance mismatch. AT 440, there is not sufficient range to make a correction but will tell you which way you want to alter the cable length to effect a correction when viewed on a Bird 43 at the duplexer output. At VHF, a 1/4" correction is not sufficient to nudge a Bird 43, but you can see it on a Bird 4381 digital meter. In any event, the stretchers are not an exact way to measure the optimum cable length because the stretchers are air line devices and have a different velocity propagation constant from coaxial cable. The same matching issue prevails on the receiver side, but in most cases, the noise floor at the site would mask any improvement that cable matching would produce at the receiver. I don't think as a discrete measure that optimizing transmitter to duplexer cables results in any improvement for the listener, but in concert with the myriad of other measures you take to optimize system performance, it's worth doing. A little bit here, a little bit there - it adds up. In a message dated 6/30/2007 3:11:19 A.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: Ian, I owe you an apology for my comment about striped tower paint in response to your 'Line Stretcher' post. I never thought of using such a tool on a short line between a transmitter and duplexer. I was thinking such was used in AM broadcast delay lines with phased towers, and never though of using such to correct such minuscule variations as might be found in a VHF or UHF duplexer and it's connection to a radio transmitter. I wonder how many dB of improvement might be achieved optimizing such minor differences and what kind of a jump in S Meter readings folks on the receiving end might realize? 73, Steve NU5D ** See what's free at http://www.aol.com.
RE: [Repeater-Builder] Cable formula
> > While the receive-side jumper may work well at any random > jumper length, the > transmit-side jumper is a different matter. My experience has been just the opposite. Unless you sweep-tune a receiver front end that uses helical resonators, the chance of having a Z even remotely close to 50+j0 are pretty slim if you just go with the manufacturers' tuning instructions. In addition, many 'stock' preamps have rather poor return loss, especially GaAsFET's that are tuned for minimum noise figure with no regard for input return loss. There was a lengthy discussion on this subject a few months back. > I think we agree that the > TX-to-duplexer jumper can be optimized, and many installers > do take the time > to try several different cable lengths. One installer I know > takes a box of > a few dozen RG-400 cables to each new install, each cable > being cut and > connectorized at 1/2-inch intervals from about 18" to about > 40". He will > keep trying cables until he finds the optimum length. What's "optimum" using this tech's approach? Maximum power output? Best efficiency? Cleanest signal? > Regarding the tuning, I always use a network analyzer to tune > a duplexer, Me too. > and I know that it is optimized for a 50-ohm source and load. > The PA is > another matter, and it seldom provides a 50 ohm match to the duplexer. You're looking at the PA being a load rather than a source. The PA can't detune the duplexer if you're looking at signal flow from the PA into the duplexer. On the receive side it's a different ballgame. > That's where a Z-matcher is a valuable tool, but- as can be > seen by several > dissenting posts- not everyone thinks it is necessary! Depends on where you draw the line. If I can't make a system perform right with all of the passives (and the actives I have control over) work with everything tuned to 50+j0, I go back to the bench, warm up the VNA, cool down a few 807's, and spend time making it right. If you can't tell by now, I'm no fan of band-aids to fix fundamental problems. Sure, many a tech has come up with an ad-hoc iterative technique to make a box play in order to get out of a site in time to be home for dinner, but let's not confuse "what worked this time" with sound engineering. --- Jeff
RE: [Repeater-Builder] Cable formula
> OK so you have a reel of cable and two connectors to make up > the jumper between transmitter and duplexer. > > The duplexer is tuned using 50 ohm test gear and the > transmitter has been optimised into a 50 ohm load. > > Unfortunatley the output impedance of the transmitter is not > 50 ohms and a length of cable to the duplexer will transform > this impedance,detuning the first can in the transmit chain. No, it won't, you're putting the cart before the horse. The duplexer and antenna system is the load. You can't change the Z of the load by changing the Z of the source. Think about it this way. Get rid of your duplexer and antenna and replace them with a fixed resistor, pick a random value, say 1000 ohms, and use a "perfect" half-wave cable between the resistor and your PA. Now use three different amplifiers to transmit into this 1000 ohm load. Does each PA have the same source Z? No. Has your load Z changed?No. Has the Z at the input to the half-wave cable changed? No. No matter what you do at the source end, the load Z is 1000 ohms and will remain that way for ever and ever Amen. Let's look at it another way using a practical example. Say you have a single bandpass cavity filter that is properly tuned to present 50+j0 at its input port when its output port is terminated with a 50+j0 load. Now you hook it up to a PA that has a strange source impedence using a half-wave cable with a Bird wattmeter somewhere in the middle (its location in terms of distance from either end is immaterial). Will the Bird show any reflected power? Of course not; the Z on the transmission line is still 50 ohms, the E and I are still in-phase at a 50:1 amplitude ratio, the cavity hasn't been "detuned" at all. The Z on the line is 50+j0, the Z at the input to the cavity is 50+j0, the Z of the load Z is 50+j0. Now, if you look in the REVERSE direction, FROM the duplexer INTO the transmitter, as you change PA's the Z looking the other way is going to vary, but that's not what we're trying to match here. If we want to go down that road, I would argue just how BAD a Z-matcher could potentially be when used on the transmit side of a duplexer in terms of how it can throw off the termination Z and mess up the RECEIVE side performance. Ponder that for a bit (hint: what should be an "open" at the antenna tee no longer is). A Z-matcher is no substitute for an isolator. --- Jeff
RE: [Repeater-Builder] Cable formula
> I don't think the cable cares whether the source and load > impedances are 50 ohms > resistive. I think the cable is indifferent to whether the > load and source values are > resistive or whether they present a complex impedance > involving +/- J. as long as the > composite value looks like 50 ohms. The cable only acts as a transformer if the *load* Z is not the same as the cable's characteristic Z. It doesn't care about the source Z; the mismatch that occurs at the source end only affects power transfer into the cable at that point. > The conventional wisdom generally expressed is that as long > as the cavities are properly > tuned, that the interconnect length from the TX is > immaterial. I question that: > > Properly tuned? When what's properly tuned and for what > parameter? Is the pass section of the cavity(s) > being tuned for maximum output or is it tuned for minimum > reflection back to the TX source? Reflection (S11). Always always always tune pass or pass/reject cavity filters for best match (pounding on the desk as I type). Too many manufacturers' instructions say to tune for maximum power transfer or least insertion loss, probably because they assume the field techs don't have equipment for measuring return loss properly. That's just not good advice in my book. If the filter is designed and built right, the insertion loss minima and return loss maxima should conincide very closely. Even if they're off a tad and you can't get them to line up perfectly, you're always better off taking 10 dB more return loss in exchange for 0.1 dB of insertion loss degradation. That primarily applies to our little narrowband two-way radio world. In wideband applications, there are other things involved, such as group delay, that come into play, but for what we're talking about, return loss is the key. > Only if the TX output impedance exactly matches the cavity > impedance and the impedance of the interconnecting cable will > the cavity tuning point be the same for either parameter. To > assume that the > TX output impedance is 50 ohms is optimistic and as you point > out, altering the power level of the > TX can affect TX output Z, the amount dependant on what TX > stages are used to control TX output. Let's straighten something out here before we get off track. Most transmitters don't HAVE a 50 ohm source Z. They are designed to work INTO a 50 ohm Z. They have internal matching transformers (stripline or otherwise) to convert the very-low-Z output of the bipolar transistors to something approaching 50 ohms so that when it is connected to our external 50 ohm world that the devices are able to transfer power. > Considering how nit-picky forum members are about designing > and building their systems, > (and I mean that in the best sense of the word), it seems > inconsistent to be indifferent to > how the duplexers might be affected by inserting what is > potentially a radical impedance > transformer between the TX and the cavities. In the absence > of any way to measure any > source and load mismatch, using a 1/2 wave (or half wave > repeating) cable length will at > least keep any existing mismatch status quo. It won't improve > the match but at least it won't > increase a mismatch because the 1/2 wave length simply > repeats the TX output Z and does > not act as a line transformer. But as the cable length > departs from a 1/2 wave and approaches > a 1/4 wave, the game changes and a 1/4 wave interconnect > between a mismatched source > and load can produce some eye opening shifts in the impedance > reflected to the load and > back to the source. Whether you have a half-wave or a quarter-wave cable terminated by a mismatched load, the VSWR remains the same. As as a simple example, assume the Z of the duplexer is 100 ohms. If you use a half-wave cable, the PA sees 100 ohms, a 2:1 VSWR. If the cable is a quarter wave, it transforms the 100 ohms to 25 ohms, again a 2:1 VSWR. Yes, the Z is not the same, but the VSWR is. You don't know whether the PA will be better off looking into the 25 ohm load versus the 100 ohm load, so why would you hold fast to the half-wave rule? For a given load Z, the VSWR remains constant no matter what cable length you use. A 50 ohm cable can't transform a non-50 ohm load to 50 ohms; it can only roll you around the Smith Chart at a constant VSWR, that being something other than 1:1. Round and round the Smith Chart we go, where she stops, nobody knows. Point being, if there is a mismatch, using a half wave cable does nothing to improve your chances of making your PA happy any more than would a quarter wave cable or any other random length. Without knowing the actual impedences involved, your odds of making an improvement using an X-length cable (pick your favorite value for X) are 50/50, nothing more, nothing less. Also keep in mind that the transformation the cable does in the case of a load mismatch is, for all practical purposes
Re: [Repeater-Builder] Line Stretcher
I whole heartedly agree, Glenn, but with one say 100 Watt pa feeding a cavity duplexer, there is no phase delay issue, just a minor mis-match, that I doubt would ever be seen or heard outside the transmitter shack. The advice I was given on duplexers is once they are tuned properly, leave them alone. The difference you see from a little tweaking will not be noticeable, and external matching devices like a Z-Matcher or a tuning stub don't generally offer enough benefit to be worth while. 73, Steve NU5D On 6/30/07, Glenn Little WB4UIV <[EMAIL PROTECTED]> wrote: WE also use a line stretcher in TV. When you have two UHF PAs that are fed to a combiner, it is very important that the phasing of the signals be correct. We use a line stretcher in the output of the exciter to the closest PA to effectively place the PAs at the same electrical distance from the exciter. This gets the phasing correct into the combiner. When dealing with 25KW PAs, a little phase difference makes for a lot of loss in signal in the combiner. 73 Glenn WB4UIV -- Ham Radio Spoken Here.NU5D Nickel Under Five Dollars
Re: [Repeater-Builder] Line Stretcher
WE also use a line stretcher in TV. When you have two UHF PAs that are fed to a combiner, it is very important that the phasing of the signals be correct. We use a line stretcher in the output of the exciter to the closest PA to effectively place the PAs at the same electrical distance from the exciter. This gets the phasing correct into the combiner. When dealing with 25KW PAs, a little phase difference makes for a lot of loss in signal in the combiner. 73 Glenn WB4UIV At 05:09 AM 06/30/07, you wrote: >Ian, I owe you an apology for my comment about striped tower paint in >response to your 'Line Stretcher' post. I never thought of using such a >tool on a short line between a transmitter and duplexer. I was thinking >such was used in AM broadcast delay lines with phased towers, and never >though of using such to correct such minuscule variations as might be >found in a VHF or UHF duplexer and it's connection to a radio >transmitter. I wonder how many dB of improvement might be achieved >optimizing such minor differences and what kind of a jump in S Meter >readings folks on the receiving end might realize? > >73, Steve NU5D > > > > > >Yahoo! Groups Links > > >
RE: [Repeater-Builder] Cable formula
> However, the load impedance of most PAs > will vary > significantly with the drive level, I think you meant source impedance. > and the input impedance > of a duplexer > cavity is always reactive Not necessarily. You can tune a duplexer very close to 50+j0 at the pass frequency. It's at frequencies off the center frequency that the selective nature of the duplexer makes it appear as a Z other than 50+j0. > However, a > simpler approach is to install an impedance matching device, sometimes > called a "Z-Matcher", at the output of the PA and adjusting > it for maximum > forward power. I disagree with this and feel it is bad advice. The point where the transmitter makes maximum power may occur at some load Z other than what the transmitter was designed for. Just because you can eek out a few extra watts by futzing with the Z-matcher doesn't mean you've done anything to improve the stability of the amplifier, nor is it guaranteed to be operating at maximum efficiency, nor do you know if the increase in power you're seeing is due to new spurious/oscillation products being generated due to a bad match. If there is a means of adjusting the loading on the PA via a Z-matcher, be it internal or external to the PA, it should first be set "flat" at 50+j0 on a VNA or return loss bridge, then hooked up to the PA, and while monitoring current draw, power output, AND spectral purity, make minor adjustments if necessary to find the best balance. The Z-matcher itself performs as if it were a narrow-band device in the sense that it will only correctly match at a single frequency. The load Z of the duplexer, which varies as you get away from its tuned pass frequency, is going to present a varying load, which the Z-matcher is going to transform again, maybe for the better, maybe for the worse. So, at best, the Z-matcher is a band aid in my book. I would much rather see an isolator with a very good input return loss on the output of the PA if there are problems that can defintively be proven to be caused by the load Z of the duplexer and antenna system. I've always maintained that if you have amplifier stability problems or problems achieving rated specs for the PA (power out, current draw, spurious, etc.), then you've got fundamental problems that need to be fixed either in the antenna system or the amplifier itself. --- Jeff
Re: [Repeater-Builder] Cable formula
OK, There was a long and detailed thread about z matchers on this group abou a year ago? Lots of interesting stuff about line matching emerged.. dont take my word for it have a look in the archives... I can only describe what I measure and that is cable leakage from a jumper between the transmitter and the duplexer when a DB products z matcher was used. (The z matcher was very nicley made with gold plated piston caps etc.) The cable leakage stopped when the z matcher was removed and the cable length was altered for optimum. Ive just dug out my line stretcher : "874-lk20L constant impedance adjustable line- general radio USA" no striped paint here im afraid! Ian Ashford G8PWE - Original Message - From: [EMAIL PROTECTED] To: Repeater-Builder@yahoogroups.com Sent: Saturday, June 30, 2007 3:50 AM Subject: Re: [Repeater-Builder] Cable formula A good quality Z-Matcher has isolation caps on the trimmer ports so I don't think the matcher itself is producing any RF radiation. I don't understand your description of the z Matcher as introducing any mismatch. The mismatch is already there as a result of some disparity between the source, load and cable impedances. All the matcher does is permit you to match the source and cable impedances. In a message dated 6/29/2007 4:40:45 P.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: The z matcher is another option but in my practical experience it makes the jumper radiate RF you spend all that money on RG214/RG400 double silver plated shielding and then deliberatley mismatch it? Ian Ashford G8PWE -- See what's free at AOL.com.
[Repeater-Builder] Line Stretcher
Ian, I owe you an apology for my comment about striped tower paint in response to your 'Line Stretcher' post. I never thought of using such a tool on a short line between a transmitter and duplexer. I was thinking such was used in AM broadcast delay lines with phased towers, and never though of using such to correct such minuscule variations as might be found in a VHF or UHF duplexer and it's connection to a radio transmitter. I wonder how many dB of improvement might be achieved optimizing such minor differences and what kind of a jump in S Meter readings folks on the receiving end might realize? 73, Steve NU5D
