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A99 higher SWR in CB band, but not in 'C' band
- Thread starter mr_fx
- Start date
went with 45 ish feet of RG8x (was running 65+ feet of RG8u)
good drop in SWRs
however not flat... but under 1.5
weird thing... added an antenna switch AFTER the SWR meter and an additional 2-3 feet of coax and the SWRs went completely flat...
So make the load on the end of that feed line (the antenna) the same impedance as the feed line (and the rest of the antenna system) and that SWR won't change no matter it's length.
That's the simple answer. If you want to know 'why', then it get's complicated because impedance is composed of resistance and reactance. There are various compositions of resistance and reactance that will 'equal' the desired impedance and still not be the 'ideal' 50 ohms and zero reactance desired. One 'catch' is that an SWR meter can't tell the difference between resistance and reactance, it can only indicate/display the combination of the two.
RF is alternating current. ALL alternating current has impedance and therefore resistance and reactance.
If it ain't one thing, it's another, right?
- 'Doc
That's the simple answer. If you want to know 'why', then it get's complicated because impedance is composed of resistance and reactance. There are various compositions of resistance and reactance that will 'equal' the desired impedance and still not be the 'ideal' 50 ohms and zero reactance desired. One 'catch' is that an SWR meter can't tell the difference between resistance and reactance, it can only indicate/display the combination of the two.
RF is alternating current. ALL alternating current has impedance and therefore resistance and reactance.
If it ain't one thing, it's another, right?
- 'Doc
ok when you go for new coax, try for lmr400 but if thasts to expensive try for rg-213.
when you buy it try to buy it in a length that is multipels of 18 feet times the velocity factor(vf).
the vf for rg213 is 66% and for lmr400 is 85% so use those numbers
(.66=12 feet or .85=15 feet 3 5/8 inches) and cut it only a couple inches longer then the number of multiples of those velocity factored 1/2 wave lengths that you need to reach your antenna.
that way the swr meter or radio will see the antenna instead of the coax.
with 1/2 wave multiples the equipment thinks its hooked directly to the antenna and you get the most honest reading of whats really going on at the antenna
even electrical half wave coaxs will REPEAT the SWR reading,............... but,............. where did you come up with the 18 feet multiple from?
18 feet times the coax velocity facter is how you get the electrical half wave for cb because its a 18 foot half wave at cb frequenceiseven electrical half wave coaxs will REPEAT the SWR reading,............... but,............. where did you come up with the 18 feet multiple from?
That "492 x VF / f = electrical 1/2 wave length" is the way to do it. 18 feet has no relation to an electrical 1/2 wave length for any typical coax on the market. That length is a length that most manufacturers found to be 'right' for a typical mobile antenna's feed line. It has nothing to do with wave length, it does have to do with the typical run distance between a mobile antenna and the radio, plus a couple of feet just in case. That 18 feet thingy has no 'special' significance at 27 Mhz but people have sure made it one'a them "magic" numbers that's supposed to 'cure' almost everything.
- 'Doc
- 'Doc
or you could start with the metric speed of light (3e10) then divide by freq in Hz, then divide by 2.54 to get inches, then divide by 12 to get feet, then divide by 2 to get a half wave in feet for the band this question and thread is about.
if you do all that or your version of the same basic formula instead of using the shortcut of 18 feet will you get a significantly different result for all the extra effort? :scratch head:
i guess its good to know the formula but i was just giving him the best simplist answer and im reasonably certain that most people on this forum know that 18 feet is right about a dead on 1/2 wave for cb so it gives a place to start calculating with the vf
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B
BOOTY MONSTER
Guest
keep in mind that in order to get that mirrored reading from the antennas feed-point in a run of coax it needs to be a electrical half wavelength (or multiple) and knowing the velocity factor and frequency is critical in calculating that length . just using a physical half wavelength doesn't mirror the reading . and yes i agree using a single or multiple electrical half wave length of coax is cutting hairs and not necessary . however , the additional loss of having a few to several extra feet of coax on the CB band (this is the CB forum section) will never ever be noticed in the real world .
BTW once you go up or down one channel from that perfectly calculated electrical half wavelength it isn't perfect anymore . and the farther away you go the more error there is . IF someone wants to do it i'd suggest doing it on the channel you use the most .
BTW once you go up or down one channel from that perfectly calculated electrical half wavelength it isn't perfect anymore . and the farther away you go the more error there is . IF someone wants to do it i'd suggest doing it on the channel you use the most .
492 divided by 27.2 = 18.08 feet and thats with your formula doc. that sure looks like a one and a 8 to me. with a little less then point one over and thats real close to dead on 18. youd never know the difference
The CB BS idea about 1/2 wave tuned feed lines showing a mirror image of what the impedance is at the other end of the line...is erroneously based on what may be good feed line theory. As meaningless as a tuned 1/2 wavelength line is to the process and success of tuning an antenna with an unknown load and resonance, you guys can't even agree on what a 1/2 wavelength long line is to start.
We're just lucky that Mother Nature doesn't require us to have everything perfect in order to work our radios.
BTW, this thread has gone all over the map trying to explain a solution to a problem and misdirection by the author.
We're just lucky that Mother Nature doesn't require us to have everything perfect in order to work our radios.
BTW, this thread has gone all over the map trying to explain a solution to a problem and misdirection by the author.
its good practice to cut feed lines at a multiple of an electrical 1/2 wave - is what a friend told me. hes a broadcast enginere who builds and maintains radio and tv stations.
it seems fairly easy to use the velocity factor and the formula for a true 1/2 wave on $3 hand calculator to figure out the coax lehgth needed.
i dont see why its an issue at all
it seems fairly easy to use the velocity factor and the formula for a true 1/2 wave on $3 hand calculator to figure out the coax lehgth needed.
i dont see why its an issue at all