Gary, Transforming from a load to 50 Ohm resistive depends on the load impedance. Same for other impedances.
Going from a load to another resistive load will always be a the ratio of N:50. So 50 Ohm coax could transform say a 75 OHM load to a 75:50 or 50:75 ratio. 75 Ohm load with 1/4 wave 50 Ohm coax to 33 Ohms, but not anything. At 1/2 wave back to 75 Ohm. In between would be R with j component, inductive or capacitive depending on length. A Smith Chart shows this. So converting the load to what you want would normally require a different cable impedance. The wiring harnesses used in multi-element antennas like the DB224 4 bay dipole antenna use cable type to convert each antenna load to a 50 Ohm input. In this case there are actually 3 harnesses, one for each of 2 dipole sets and then a 3rd to take these two to one input. I think 92 or 62 Ohm cable is used, but not sure. 73, ron, n9ee/r >From: Gary Schafer <[EMAIL PROTECTED]> >Date: 2007/08/27 Mon PM 10:52:11 CDT >To: Repeater-Builder@yahoogroups.com >Subject: RE: [Repeater-Builder] Duplexers > > > > >From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of >Jesse Lloyd >Sent: Monday, August 27, 2007 9:39PM >To: Repeater-Builder@yahoogroups.com >Subject: Re: [Repeater-Builder]Duplexers > >If your coax is the sameimpedance as your transmitter, but different than your >load, can it still be atransformer though? Is it possible to transform a >load that isn't 50 ohmsto 50 ohms using 50 ohm coax? > >Yes italways acts as a transformer when the load impedance is not the same as >thecoax impedance. >You cannot transform any impedance to 50 ohms with a 50 ohm cable. You can >transformto something above or below the 50 ohm cable impedance. >Thereason changing the length of the coax to a transmitter helps sometimes, >eventhough the transformation of impedance is not to 50 ohms, is that >thetransmitter may see an impedance that it is happier with than what the >originaltransformed impedance was. > > >Yes your right VSWR is the ratio between Vmax and Vmin, node and anodes, of >theinterference pattern caused by standing waves. Even still there is apoint >where the voltage is at a minimum on the line. What happens if thatpoint is >at the transmitters output... does it help keep the heat down in >thetransmitter due to high SWR? > >It doesnâtmatter where the min and max are on the line. The same amount of >reflectedpower will be seen at any point. Reflected power does NOT get back >into thetransmitter. It gets re-reflected back towards the antenna when it >reaches thetransmitter circuits. >If youhave a 100 watt transmitter with 10 watts reflected from the load >yourwattmeter will read 110 watts forward and 10 watts reflected. The extra >10watts forward power comes from the 10 watts that is reflected from the load >andre-reflected at the transmitter. The re-reflected power adds to the >original100 watts forward power for a total of 110 watts forward power. All of >the 100watts eventually gets radiated by the antenna. This is of course >disregardingany line loss which would lower the reflected power indication by >the amount ofline loss. Line loss would also claim a portion of the >re-reflected power too. >If youhave two watt meters and an antenna matching device you can put one >wattmeter betweenthe transmitter and the matching device and tune it for >minimum reflected poweron the first meter. Then with a second meter between >the tuner and themismatched load you can see the second wattmeter that is >reading the reflectedpower. The second wattmeter will have a higher forward >power reading than thefirst due to the added re-reflected power. > >With amismatched load the transmitter may run hotter because it is under >oroverloaded due to the non 50 ohm load that it is seeing but it is >notdissipating any of the reflected power. Many solid state transmitters >aresensitive to reactive loads and may draw more current because of this. > >73 >Gary K4FMX > > > > >On 8/27/07, GarySchafer <[EMAIL PROTECTED]>wrote: >VSWR (voltage standingwave ratio) will be the same at any point on a >transmission  line. Theimaginary standing wave does not move as the forward >and reflected power does.The voltage standing wave ratio is the ratio of the >forward voltage to thereflected voltage at a given point on the line. As you >move up or down the linethe forward voltage will change and so will the >reflected voltage but the ratioor difference between the two will work out to >the same value. Thus the term"standing wave". The wave appears to stand still >on the line as itoscillates up and down in a sin wave manor. > >As Jeff has said theimpedance shown to the transmitter will be different with >different lengths oftransmission line only if the load is not a perfect 50 >ohms assuming a 50 ohmline. With a load that does not match the line the line >operates as animpedance transformer. Think about what a quarter wave length >line looks likewith a short on one end. It transforms that short to a high >impedance or openat the other end. If one end is open the other end will look >like a short tothe transmitter. >With a load impedancethat is not 50 ohms what is seen at the transmitter is >something between anopen and a short depending on how far from 50 ohms the >load is. In other wordsthe load impedance gets "transformed" to something else. > >73 >Gary  K4FMX > > >From: [EMAIL PROTECTED]:[EMAIL PROTECTED] Behalf Of Jesse Lloyd >Sent: Monday, August 27, 200711:48 AM >To: Repeater-Builder@yahoogroups.com >Subject: Re: [Repeater-Builder]Duplexers > >The length of coax doesn't effect impedance. Trimming the coax effects what >is read on the VSWR meter because what isactually happening is that there is >an interference pattern created when youhave a mismatch on the end of >feedline. This pattern is sinusoidal andchanges in voltage and current along >the line, in 1/2 wave periods. Youwill find max voltage peaks and min >voltage peaks. Also current will goup and down too. When you are using a >VSWR meter you are measuringvoltage, if you move the meter to a different spot >on the cable, the voltage isdifferent, therefor it gives you a different >reading. > >Now if you put a voltage null at your transmitter, what would >happen? Normally with high SWR your transmitter will get hot because its >dissipatingthe reflected power into its heatsink. If you put it at a voltage >null, Iwould suspect that the SWR would not get dissipated by the transmitter >as muchas if you put it at a voltage peak. The standing waves are still >there,there is still a mismatch, you will get the same power out, but its just >notgoing to hurt your transmitter as much because of the heat. > >The only time coax length makes a difference to power out is if your using >itin a matching stub, or a matching section ie. if you take 1/4 wave of 75 >ohmcable put it on the end of 50 ohm cable you will get a match with a 112.5 >ohmload. > >You make an interesting point though, why does the cabling of duplexer's >needto be a certain length. I would suspect that its because they are >loopedand make an inductor. This then is part of the LC filtering, and >changing thelength effects L. But I could be wrong on that. > >Jesse >On 8/27/07, R. K.Brumback <[EMAIL PROTECTED]>wrote: >I have heard this pointargued for years. "Does trimming the coax affect the >SWR?" Ifthe length of coax has an affect on impedance, then how could it not >affectpower out? We strive to maintain 50 ohms at the tail of all devices to >matchthe end load. GE puts matching networks in their Mastr II's. I have taken >aMFJ-259 and soldered a PL259 only at one end and then started trimming the >coaxdown and watched the impedance change significantly with each cut. >Duplexerscome with precise lengths of cabling.  I have heard that trimming >coaxonly fools the meter. Not being an engineer with millions worth of >equipment Ican only make a SWAG (scientific wild ass guess) as to whether coax >lengthmakes a difference in power out. >Randy >W4CPT > >-----Original Message----- >From: [EMAIL PROTECTED]:[EMAIL PROTECTED] On Behalf Of Jeff DePolo >Sent: Saturday, August 25, 200712:30 AM >To: Repeater-Builder@yahoogroups.com >Subject: RE: [Repeater-Builder]Duplexers > >> When you put the Bird between the TX and theduplexer, you >> have changed the >> length of the jumper cable, which upset the tuning. > >Adding a wattmeter or any other length of cable between the transmitter and >the duplexer Tx input port has no effect on the tuning of the duplexer. It >may change the load Z the transmitter sees, which may make the transmitter >happier (or sadder) depending on the resulting Z, but in no way does it >alter the tuning of the duplexer itself. > >Adding or removing cable lengths between the transmitter and duplexer also >does not change the VSWR as seen by the transmitter (minimal cable loss >effects notwithstanding). > >--- Jeff > >----------------------------------------- >Jeff DePolo - [EMAIL PROTECTED] >Broadcast Sciences LLC, Valley Forge PA >v: 610.917.3000 >f: 610.917.3030 > >No virus found in this incoming message. >Checked by AVG Free Edition. >Version: 7.5.484 / Virus Database: 269.12.6 - Release Date: 8/24/2007 12:00 AM > >No virus found in this outgoing message. >Checked by AVG Free Edition. >Version: 7.5.484 / Virus Database: 269.12.6 - Release Date: 8/24/2007 12:00 AM > > > > > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
