Never did care for algebra myself. I had to use it in my work but I never liked it.
Oh.....trigonometry too! More fun not to like!
LOL
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70cm folded dipole
- Thread starter Leland
- Start date
Never did care for algebra myself. I had to use it in my work but I never liked it.
Oh.....trigonometry too! More fun not to like!
LOLI love the half wavelength multiple myth...
This:
What a half wavelength multiple of feedline does is present the exact impedance that is present on one side of the feedline (the antenna in this case) to the other side of the feedline (whatever hapens to be plugged into the other end, radio, VNA, MFJ Analyzer, SWR meter, whatever). This would be useful if you were, say, using 75 ohm coax instead of the 50 ohm coax your radio would expect. This is also useful when tuning the antenna with something more advanced than a simple SWR meter. As far as your radio is concerned, it only cares about the SWR that it sees, and as long as that is below 2:1 (any maybe even higher in some cases) its happy.
The half wavelength multiple feedline myth has been around for a long time. Even with a mono-band antenna using such a length doesn't benefit you any.
The DB
On my X-Dipole antenna project,I'm thinking I have to have the main feed line measured out in half wavelengths along with the phasing cable being exactly at 1/4 wavelength for one dipole and 1/2 wavelength for the second dipole. This is critical in order to make circular polarization work properly.
HUH??? aparently, you didn't read what I posted.
"to answer that part of the question,...... yes, feeding an antenna @ elect 1/2wl mults is the same as full wl's."
... and it is.
wheres the "myth"?:blink:
Ok. Oops. Yes I need to reread it. I was busy at work and what DB said caught my eye. Apologies.
Yes I agree with you 100%. Either in this thread or another somewhere in the last day or so I mentioned that I simply run whatever it takes to reach and only use tuned 1/2 wave feeders when initially tuning an antenna with my antenna analyzer. The only time you need precise 1/4 or 1/2 wavelengths (or whatever length) is when dealing with phasing or impedance transformation.
Apparently you misunderstood my post...
As a full wavelength is a multiple of a half wavelength they will work the same, no argument there, and none was intended.
The myth I was referring to is the need of (or for that matter any assumed benefit from) the use of any specific length of feedline (half wavelength multiples included) for regular operation. This assumes that the feedline is the proper impedance, phasing is not being used, or one of the other very rare exceptions is not in play.
The DB
You and I would do real well in a discussion about installing mobile CB antennas then as they all seem to think you need 18 feet of coax and ONLY 18 feet of coax. Don't even get me started when someone asks "How much cable do I need between my radio and my amp to get good SWR's".
Note: Plural use of SWR was intentional.
In Sirio's email to me about inquiring about the 12' coax length on their PL145 magmount,they replied with saying that there was no reason for that length other than that's what they decided without any reference to 1/4 wavelength. The magmount is the one I bought to go with my Sirio Performer PL-5000
Well that's one case of honesty. The issue seems not to be with the manufacturers mostly but rather with the operators who seem to follow internet folklore and swear by the 18 foot rule of most installations. Most of these same people further quote that it is 1/2 wavelength long and thus repeats the actual antenna impedance. That is where they fall flat on their [slash]ass[/slash] face because they do not take into account the velocity factor of the cable. Most do not even understand what the velocity factor is or does to lengths. Not all mind you but this does apply to a great deal of folks that simply read and repeat without learning something in the middle.
You and I would do real well in a discussion about installing mobile CB antennas then as they all seem to think you need 18 feet of coax and ONLY 18 feet of coax. Don't even get me started when someone asks "How much cable do I need between my radio and my amp to get good SWR's".
Note: Plural use of SWR was intentional.
To answer that question "someone" asked, "yes" works well...
About the discussion on installing mobile CB antennas, I've read many of your posts since I started here and I doubt we would have much of a discussion as our facts seem to line up well.
The DB
To answer that question "someone" asked, "yes" works well...
About the discussion on installing mobile CB antennas, I've read many of your posts since I started here and I doubt we would have much of a discussion as our facts seem to line up well.
The DB
OH......that's what I meant. You and I against all the 18 foot believers.
OH......that's what I meant. You and I against all the 18 foot believers.
Their beliefs are unshakable to the point of fanaticism... Perhaps we should start calling them the "cult of the magic length"...
So if the goal is a low SWR I suppose we could always recommend 1000 feet of RG-178. They will have a 1:1 SWR clear from one end of the radio spectrum to the other... That should make them happy right? After all, it is what they asked for...
Leland, we all start somewhere. If it doesn't work to expectations learn from the experience and move on. Don't be afraid to ask questions, both in what to do and what went wrong... There are many very knowledgeable people on this forum, even if we do sometimes bicker a bit... A good reference book never hurts either, such as the current version of the ARRL Antenna Book (22'nd edition at the moment). I know its a bit pricey for some, but that book was the beginning of real understanding for me, and I have quoted from it here many times.
The DB
agree on that,..... the CB "18 foot" thingy is a myth,
however, with certain antennas, (end fed Zeps for instance) it does make a difference if it is fed from a Voltage loop or a Current loop
- No one is chatting at the moment.
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