Just reading, at 3:30 am... what's this about antenna impedance? seeing as i'm buying a decent transmitter I figure I had better have a decent antenna that isnt going to wreck it. The antenna I planned to use is a Jpole but I can't seem to find the impedance anywhere. The impedance I need is 50 ohms and i have no clue how to test this.
There are many good J-Pole design plans on the web for reference. Simply build one for your operating frequency and then peak the tuning of the antenna by sliding the connection points equally up or down for minimum VSWR. Do this with low power to get close to the peak point, then gradually raise power and re-peak the antenna for minimum VSWR. It will take a few times as J-Pole antennas are not renowned for their quick or easy setup. They take a lot of adjustment and patience to get them set up right.
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
okay, are VSWR and SWR the same things? I have my SWR tuned 1:1, but I just want to make sure my antenna is getting 50 ohms.
Yes. The "V" stands for "Voltage", and "SWR" is "Standing Wave Ratio"....so all together now..."Voltage Standing Wave Ratio"....all means the same thing.
A 1:1 VSWR/SWR is excellent! And if your TX has a 50 ohm output impedance along with the coax being 50 ohms, and your antenna is connected properly and tuned properly, with a 1:1 ratio...your system is loading at 50 ohms...plus or minus an ohm or two.
Keep in mind that this impedance loading is AC and not DC impedance. Measuring across that coax connected to a J-Pole will obviously indicate continuity on an ohm meter. But the J-Pole at the AC frequency of your transmitters operating frequency appears to it as a 50 ohm match, thus the 1:1 VSWR/SWR ratio.
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
On a side note it is neat that J-Poles allow the user to position the pipe clamps so to change impedance. Even though most transmitters expect a 50 ohm load, it is possible to tune a J-Pole to 75 ohms, and even 300 to 450 ohms. Makes me wonder if twin lead coax or ladder line could be used on a J-Pole instead of regular coax? If the transmitter had the ability to properly load up to 300 or 450 ohms that would make for some almost loss less line. Maybe that wouldn't work though? Just passing by an idea. Also 75 ohm coax is so damn cheap it would be useful to use in quick and dirty situations and I do believe most lower powered transmitters can match up to 75ohms w/o too much issue even if they were designed for 50.
Makes me wonder if twin lead coax or ladder line could be used on a J-Pole instead of regular coax? ....and I do believe most lower powered transmitters can match up to 75ohms w/o too much issue even if they were designed for 50.
I have done both twin lead and 75 ohm coax methods. All that is needed is an impedance transformer for the twin lead approach.
For the 75 ohm route, there shouldn't be too much headache but that depends on the transmitter's output circuitry. Even a 25 ohm difference might be enough to throw off a transmitter's harmonic filter performance, or cause the finals to operate a bit out of peak efficiency, perhaps run a little warmer.
The thing we want to prevent is an often forgotten and sometimes not even considered, is "Internal VSWR" between the finals and output filter due to changes of loading from the designed parameters to something different without proper impedance matching.
Rarely is a transmitter wired up to measure for VSWR between the final stages and the output filtering circuitry. But the fact is, high VSWR can develop at this juncture even though an external VSWR meter indicates a good match to the antenna. Coupling efficiency and RF energy transfer may be affected if the transmitter's output circuitry was designed to have a narrow window of operating parameters or if the output circuitry has inadequate coupling and isolation factors...ie poorly designed and less than optimum components used.
Most 5 pole and up filter networks can tolerate a fairly wide loading swing and the use of tolerant finals. The key however is to recognize that the output filter, if designed to operate in a 50 ohm environment, may loose its harmonic suppression performance, or make the harmonics worse or cause parasitic oscillations in the finals....often leading to POOF in high powered systems or cause a spectrum full of unwanted junk...not to mention a plethora of unwanted complaint attention!
VHF is very picky about impedance matching...UHF and above even more picky. Solid state components, though these days tend to have a fairly wide tolerance even up to and including direct shorted outputs, will continue to operate and survive. However even if this is the case, the spectral purity is important here because not just of preventing unwanted attention, but to prevent your final amplifier stage from splitting up its amplification ability across the spectrum throwing out a bunch of harmonics instead of being focused on the frequency intended to be transmitted..in effect robbing power from your carrier...making it weaker.
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
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