RadioBanter - View Single Post - Matching on the MFJ-1800

Jeff Liebermann[_2_]

On Thu, 19 Nov 2009 16:20:21 -0800, (Dave Platt)
wrote:

In article ,
Jeff Liebermann wrote:

Thanks. 50 ohm coax does not make it a matching section to a 200 ohm
folded dipole. However, the ferrite beads are a good way to simply
block the reflected power from the folded dipole so that it looks like
it's matched. In any case, that reflected power is lost (converted to
heat) in the ferrite beads.

Ummm... I'd disagree (or at least quibble) on at least two grounds.

Oh-oh.

Ground #1: the ferrite beads would only block non-balanced current
flow back down the outside of the coax. They will have no effect at
all on power which is reflected back down the inside of the coax (the
center conductor and the inside of the shield) from any impedance
mismatch where the coax meets the folded dipole.

Agreed, if there were a balun present. Without a balun, there's going
to be some radiation from the coax cable, presumably from the shield.
Disclaimer: I don't fully understand how baluns work, and certainly
don't understand what the piece of coax with ferrite beads on the
MFJ-1800 is suppose to do.

In effect, the presence of the beads (if they're choking the coax
properly) actually ensures that the transmitter *does* see the true
effect of any impedance mismatch. The transmitter is just as likely
to see a higher SWR than a lower one, when the beads are added.

You lost me here. Just what are the beads suppose to do if not block
the reflected signal? Presumably, they do serve some useful purpose.
Now, you're suggesting that they could make the VSWR worse?

Ground #2: the beads do not necessarily result in a significant loss
of power. If their RF impedance is high enough at the frequency of
use, then RF current flow through them will be negligible, and there
won't be a loss of power. Power loss in choke-ferrites tends to be
worst when the RF impedance is both resistive, and too low for the
application (i.e. still allows substantial current flow, which then
results in dissipation of power inside the choke).

This is beginning to sound like the discussion about whether it's
acceptable to use a high VSWR antenna. The standard answer is that if
the transmitter can take it without going into protection or
oscillation, then it's acceptable. I don't have a problem with that,
although I've never seen a Yagi-Uda antenna with a 50 ohm coax hung
directly onto a folded dipole because there are usually easy ways to
do the matching and balanced to unbalanced conversion.

Digging a deeper hole, I've been assuming that if the ferrite beads
were not there, the coax cable will radiate. After all, that's one
purpose of a balun, to prevent coax radiation from mangling the
pattern. That's still a dubious proposition due to the large length
of exposed center conductor at both ends of the coax piece, which
certainly will radiate some. I can add that to the model, but I don't
know how to model the ferrite beads.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558