"Dave" wrote in message
. ..

"Cecil Moore" wrote in message
om...
Dave wrote:
"Cecil Moore" wrote:

Owen Duffy wrote:
This has nothing to do with the stated myth: Measurements with a Bird
43 of the conditions on the Thruline section are invalid unless it has
some minimum length of 50 ohm line on both sides of itself.

Would you be willing to make the same statement about an MFJ wattmeter?

now your are just trying to muddy the waters... i wouldn't trust an mfj
to measure anything!

No, I'm just trying to get back to the original discussion which
was: Do SWR meters need 50 ohm coax surrounding them to establish
the assumed 50 ohm environment? The majority of SWR meters have
no Thruline and MFJ seems to make no attempt to establish a 50
ohm environment like Bird does. What is the Z0 of a meandering wire
surrounded by an aluminum box?

The original discussion (V/I ratio is forced to Z0) had nothing to
do with Bird wattmeters. The original discussion was about SWR meters
in general (which the Bird is not). The mythbusters thing was an
interesting diversion away from the original question which remains
unanswered.
--

an interesting diversion until everyone was convinced that you were off
the wall. now its time to re-open a dead thread???.

as far as cheap swr meters, the daiwa, swan, and mfj manuals all require
50 ohm coax 'for accurate readings'... a joke by any standard of
measurement for that type of instrument. but just to put this one to rest
quickly... i set up my tdr and ran some quick measurements. this tdr will
resolve a 6" 75 ohm jumper in the 25' or so of 50 ohm test cable that i
used. i measured an mfj-815b and a daiwa ns-660pa and they are
indistiguishable from the 50 ohm line. so the answer is yes, they do
internally look like a 50 ohm line section.


ah, good cecil hasn't replied yet... sorry, i should have put this in the
same message, but the sun was trying to come out and i had to take advantage
of it. we haven't seen the sun here for the last 10 days which is the only
reason i was bored enough to bother getting into one of these 'discussions'
again.

now for the other part of the story. cecil states that a 50 ohm
'environment' must be established around these meters to get proper
readings. this 'environment' requires some unknown length of 50 ohm cable
on both sides of the meter. i can disprove this one easily... i again used
the two meters above. first, attach 50 ohm dummy load to the meter, set
transmitter to 100w forward on the meter, read reverse... there is always
some small movement, but not much more than a needle width on either one.
now, get 1/2 wave of 75 ohm cable, attach between the dummy load and the
meter antenna output... again adjust for 100w forward and voila, same needle
wiggle as before. what does this mean? well, if indeed the meter was
relying on the length of 50 ohm coax on the output to 'establish the
environment' as cecil seems to think is necessary then the 75 ohm cable
should have upset this 'environment' and caused an incorrect reading...
since the reading was correct for a 50 ohm meter then it must be that the
required length of 50 ohm cable is zero.

now of course cecil will rant and rave and say the meter is reading
incorrectly since there is obviously some reflected power in the 75 ohm
cable. and he will be correct on that point, however, once you change the
reference Zo to 50 ohms inside the meter there is no reflected power to
measure since the load presented at the end of the 75 ohm line is 50 ohms.
QED.