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1-9 of 9 messages
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Antenna feedpoint impedance
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by AC7WL on August 20, 2003
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Possibly a dumb question, but I'll ask anyways... I've read in various articles that although you can match a transceiver to your feed line/antenna with a transmatch, your antenna will still have an impedance mismatch at the feed point. For example, I currently feed my inverted V dipole with 450 ohm ladder line. I use an MFJ 962C antenna tuner to match my transceiver to the this feed line, yet I would still have a mismatch at the feed point owing to the roughly 50-60 ohms at the feed point of the antenna and the 450 ohm impedance of my ladder line. Now, my setup works quite well, and my dipole cut for 40 meters hits em' solid up and down the West coast QRP on that band. Am I incurring losses in this setup due to using the ladder line vice coax which would be matched more closely to the antenna's impedance? I suppose I could yank my ladder line and replace it with coax and a current balun, but would like some more info on this. Any and all info would be appreciated.
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RE: Antenna feedpoint impedance
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by WB2WIK on August 20, 2003
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You didn't say how long your inverted vee dipole is.
If it's 65-66' long overall, so it's really a 1/2-WL doublet on 40m, and you only use it on 40m, I would surely replace the ladder line with good-quality coax if for no other reason than to have less knobs to twist (no need for the antenna tuner).
However, if the doublet is a non-resonant length, or you use it on multiple bands, the use of ladder line and a tuner is likely far better.
For example, if you have a 40m inverted vee but want to use it on 20m, feeding it with coax won't work. Why? Because it's a one-wavelength center-fed, which means it's "voltage fed" on 20m (rather than current-fed, as it is on 40m), so it's feedpoint Z is sky-high -- far higher than 450 Ohms, and feeding it with coax results in such a gross mismatch that you'd be lucky to even hear anybody with it, let alone make contacts.
So, again, the transmission line choice is and should be based entirely upon the application.
WB2WIK/6
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RE: Antenna feedpoint impedance
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by FJGH on August 20, 2003
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Since you are using very low loss ladder line to your antenna, a high swr is of little consequence. You have one of the most efficient multiband antennas ever conceived. Worry no more.
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RE: Antenna feedpoint impedance
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by KB1GMX on August 20, 2003
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Despite the mismatch of the ladder line to the antenna your losses are low due to using ladder line.
The system would be inefficient if the ladder line was replaced with some other cable that did not have the low loss characteristics of ladder line and still not matching the antenna impedence.
Replacing the Ladder line with coax would work as well but likely not better at the frequency the antenna is cut for. However, if you use that antenna for other bands the feed point is any thing but 50 ohms and ladder line and tuner will work better.
Hope that helps.
Allison
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RE: Antenna feedpoint impedance
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by K5DVW on August 20, 2003
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"if you use that antenna for other bands the feed point is any thing but 50 ohms and ladder line and tuner will work better. "
I don't agree with that blanket statement. Tuners have intrinsic losses which increase with the amount of reactance needed to perform a match. There are situations where a fairly well matched coax coupled with a tuner will slightly outperform the same tuner plus open wire feeders. This is especially so when the load impedance is low.
With the open wire line, there is also the loss of a balun to consider.
I'd not hesitate to use coax and a tuner at HF for anything less than 5:1 VSWR measured at the antenna.
Tuners and open wire line are best for multiband/broadband antennas. I sure couldnt argue to use coax in that situation.
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RE: Antenna feedpoint impedance
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by WB6BYU on August 20, 2003
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Let's get back to the original question: is his
feedline mismatched to the antenna when he uses a tuner
in the shack. I would say it is correctly matched, at
least to a first order approximation.
First let's look at how the system works on transmit.
Your rig is connected to the tuner through a 50 ohm
cable, but it will only see a 50 ohm impedance when
that is the load presented by the tuner. The job of
the tuner is to take the input impedance of the
feedline - whatever it might be - and transform it to
a 50 ohm load so the rig sees 50 ohms at the other
end of the coax. The impedance the tuner sees looking
into the 450 ohm line is dependent on the input
impedance of the antenna and how it is transformed by
the length of feedline.
Or we can think of the tuner and feedline together as
the "linear tuner" that matches the input impedance of
the antenna to the 50 ohm coax. Imagine the case of
an antenna with an input impedance of 4000 ohms fed
with 450 ohm line. If the feedline is exactly a quarter
wavelength, the impedance at the other end will be very
close to 50 ohms and we wouldn't need the extra box
in the shack - the feedline itself is the "tuner" in
this case.
So on transmit we have some "linear tuner" (the box in
the shack plus the feedline) that matches the input
impedance of the antenna to 50 ohms. Now, what happens
on receive? Our "linear tuner" that matches 50 ohms
to the antenna impedance works in both directions: if
the rig presents a 50 load to the coax, then the antenna
will see an impedance looking into the top end of the
cable that matches its characteristic impedance, and
we achieve maximum power transfer. True, the 450 ohm
feedline may still be operating at a high SWR, but
the antenna is seeing an optimum load.
The biggest uncertainty in this is the input impedance
of the receiver: this may not be particularly close
to 50 ohms, but probably close enough that peaking
the tuner for lowest SWR on transmit will be very close
to optimizing it for receive, and vice versa. (Which
is why peaking a tuner for maximum received signal is
a good practice - it can get you pretty close to the
right setting without causing any QRM on the air.)
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RE: Antenna feedpoint impedance
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by KE4MOB on August 20, 2003
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IF you only use the antenna for 40 and IF it is resonant for 40, then you MIGHT could do a little better by dropping the tuner and ladder line and going with straight coax. The improvement might be actually measurable to the receiving station, and then again, it may not.
But if you use it for multiband operation, there are more variables to consider such as:
Type of feedline
Length of feedline
SWR at antenna
Operating Frequency
I would suggest reading Maxwell's "Reflections" at
http://www.youmax.com/rcarc/bookshelf.htm
as well as
http://www.antennex.com/preview/vswr.htm
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RE: Antenna feedpoint impedance
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by AC7WL on August 20, 2003
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Thanks a bunch, I figured I had the optimum or "best solution" for my setup. I use the antenna on other bands as well (primarily 15, 20, and 80), thus the desire to use the feed line with lowest loss. I was just curious about this as it would have suggested I had a gross mismatch at the feed point of my dipole. However, I could not understand why it worked so well if this were the case.
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RE: Antenna feedpoint impedance
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by AD6JN on August 21, 2003
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For a fixed frequency the feed point impedance would be seen by the TX if the ladder line was 1/2 wavelength increment. For other frequencies the antenna tuner "in effect" adds or subtracts feedline length so the TX will see the 50 ohms.
A good experiment would be to terminate the 1/2 wave length increment feedline with a 50 ohm resistor and look at the VSWR and impedance with an antenna analyzer. You will see 50 ohms.
Your system is OK.
Bob
AD6JN
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