TV antenna impedance measurements with MFJ-269.
 

Hi Group,

Have a MFJ-269 here (new toy) and did a bit of expermienting with the TV
antenna over the Christmast weekend, I have one of those 13 element antenna
from radio shack, I don't know the exact part number.

I tied the MFJ-269 into the 300 ohm to 75 ohm balun at the antenna. The
antenna being about 20 feet up on the house peak.

Looking at the impedances I got, nothing really looked good at the TV
frequencies as I would have expected.

If the match is this bad, I would expect alot of loss from the fact that I run
75 ohm cable down from this antenna to the TV set.

Could my reading be mis-leading since the balum is a 4:1 going from 300 ohm to
75 ohm?

Has anyone done this before and posted the data? I would be interested to see.

I don't have my data in front of me at this time so I don't have any
measurements to post.

I really find the MFJ-269 an exceptional device for the antenna experimenter.

Tnx de KJ4UO

On 27 Dec 2004 14:09:32 GMT, (PDRUNEN) wrote:

Hi Group,

Have a MFJ-269 here (new toy) and did a bit of expermienting with the TV
antenna over the Christmast weekend, I have one of those 13 element antenna
from radio shack, I don't know the exact part number.

I tied the MFJ-269 into the 300 ohm to 75 ohm balun at the antenna. The
antenna being about 20 feet up on the house peak.

Looking at the impedances I got, nothing really looked good at the TV
frequencies as I would have expected.

If the match is this bad, I would expect alot of loss from the fact that I run
75 ohm cable down from this antenna to the TV set.

Could my reading be mis-leading since the balum is a 4:1 going from 300 ohm to
75 ohm?

Has anyone done this before and posted the data? I would be interested to see.

I don't have my data in front of me at this time so I don't have any
measurements to post.

I really find the MFJ-269 an exceptional device for the antenna experimenter.

Tnx de KJ4UO


Dear KJ4UO,
You might look through the user's guide and check me on this, but is
it not the case that the MFJ-269 is designed and calibrated for 50
ohms? If that is the case, as I think it is, then anything other than
50 ohms is going to look like it has high SWR. If you TV antenna is
what it is supposed to be, then looking into the 300 ohm Xfrm would
look like 300 + j0 (ideally) and if you look into the 75 ohm cable,
instead, it would look like 75 + j0 (ideally). The SWR in those two
cases would be 6 and 1.5, respectively and that's the BEST you would
get.
Bob, W9DMK, Dahlgren, VA
http://www.qsl.net/w9dmk

"PDRUNEN" wrote in message
...
Hi Group,

Have a MFJ-269 here (new toy) and did a bit of expermienting with the TV
antenna over the Christmast weekend, I have one of those 13 element
antenna
from radio shack, I don't know the exact part number.

I tied the MFJ-269 into the 300 ohm to 75 ohm balun at the antenna. The
antenna being about 20 feet up on the house peak.

Looking at the impedances I got, nothing really looked good at the TV
frequencies as I would have expected.

If the match is this bad, I would expect alot of loss from the fact that I
run
75 ohm cable down from this antenna to the TV set.

Could my reading be mis-leading since the balum is a 4:1 going from 300
ohm to
75 ohm?

Has anyone done this before and posted the data? I would be interested to
see.

I don't have my data in front of me at this time so I don't have any
measurements to post.

I really find the MFJ-269 an exceptional device for the antenna
experimenter.

Tnx de KJ4UO


You will want to measure Z, not SWR. My offhand guess is that impedances
between 25 and 225 Ohms are about as good as you could expect. There will
probably be at least one frequency between channels2 & 6 and one between
channels 7 & 13 where you get some R + j0.

You might also want to put the MFJ in the mode where it displays parallel R
& X. It might show more of a trend.

The antenna not being 300 Ohms does not result in high SWR loss in the coax.
Your receiver not being 75/300 Ohms would. This is the inverse of the
transmitting case.

Tam/WB2TT

On Mon, 27 Dec 2004 15:55:14 GMT, (Robert
Lay) wrote:

On 27 Dec 2004 14:09:32 GMT, (PDRUNEN) wrote:

Hi Group,

Have a MFJ-269 here (new toy) and did a bit of expermienting with the TV
antenna over the Christmast weekend, I have one of those 13 element antenna
from radio shack, I don't know the exact part number.

I tied the MFJ-269 into the 300 ohm to 75 ohm balun at the antenna. The
antenna being about 20 feet up on the house peak.

Looking at the impedances I got, nothing really looked good at the TV
frequencies as I would have expected.

If the match is this bad, I would expect alot of loss from the fact that I run
75 ohm cable down from this antenna to the TV set.

Could my reading be mis-leading since the balum is a 4:1 going from 300 ohm to
75 ohm?

Has anyone done this before and posted the data? I would be interested to see.

I don't have my data in front of me at this time so I don't have any
measurements to post.

I really find the MFJ-269 an exceptional device for the antenna experimenter.

Tnx de KJ4UO


Dear KJ4UO,
You might look through the user's guide and check me on this, but is
it not the case that the MFJ-269 is designed and calibrated for 50
ohms?

The 269 has an "Advanced 3" menu (page 30) that allows you to set the
SWR reference impedance to values other than 50 ohms, and measure
line loss and SWR in systems other than 50 ohms.

bob
k5qwg

If that is the case, as I think it is, then anything other than
50 ohms is going to look like it has high SWR. If you TV antenna is
what it is supposed to be, then looking into the 300 ohm Xfrm would
look like 300 + j0 (ideally) and if you look into the 75 ohm cable,
instead, it would look like 75 + j0 (ideally). The SWR in those two
cases would be 6 and 1.5, respectively and that's the BEST you would
get.
Bob, W9DMK, Dahlgren, VA
http://www.qsl.net/w9dmk

In article ,
says...
Looking at the impedances I got, nothing really looked good at the TV
frequencies as I would have expected.

If the match is this bad, I would expect alot of loss from the fact that I run
75 ohm cable down from this antenna to the TV set.

__________________________________________________ _________

The important thing with a TV antenna is broadband response across each
6 MHz channel. Signal loss can be made up with an amplifier mounted at
the antenna, but if there is severe impedance variation across a
channel, you will have a lot of ghosting in the picture. Some
frequencies will have a high SWR and others not, and that will cause
differing amounts of reflection across the band.

This same problem exists with TV transmitting antennas, and engineers go
to great lengths to make the response uniform.

--
Bill, W6WRT

On Mon, 27 Dec 2004 09:26:57 -0800, Bill Turner
wrote:
but if there is severe impedance variation across a
channel, you will have a lot of ghosting in the picture

Hi Bill,

Actually, ghosting is path problem where two signals time lag
(reckoned by distance traveled by different paths - at least 100s of
meters if not km) present the viewer with two pictures.

The impedance variation gives rise to color distortion, smearing, due
to relative phase shift and/or BW clipping.

73's
Richard Clark, KB7QHC

In article ,
says...

Actually, ghosting is path problem where two signals time lag
(reckoned by distance traveled by different paths - at least 100s of
meters if not km) present the viewer with two pictures.

The impedance variation gives rise to color distortion, smearing, due
to relative phase shift and/or BW clipping.


__________________________________________________ _________

The "ghosting" I'm talking about is a very close-spaced ghost, perhaps
1/4 inch or less on the screen. It can definitely be caused by a poor
antenna. I used to install TV antennas for a living, trust me on that.

If you're talking about a ghost spaced several inches away on the
screen, then your statement above is correct.

--
Bill, W6WRT

On Mon, 27 Dec 2004 11:38:51 -0800, Bill Turner
wrote:

The "ghosting" I'm talking about is a very close-spaced ghost, perhaps
1/4 inch or less on the screen. It can definitely be caused by a poor
antenna. I used to install TV antennas for a living, trust me on that.

Hi Bill,

For the benefit of others, let's put this into perspective. The time
it takes to scan one line across the face of a TV screen, is 50µS
which from my Radar days would have been 5 Radar-Miles. As this is
TV, instead that scan line is 10 Miles long (for RF traveling at 5µS
per mile). All this math is going to be about 5-10% off:

If we take the TV screen to be a modern size of 30" diagonal - call it
25" across, then a ghost offset by one inch has its origins in a
reflection (somewhere) that increases the path length by 0.4 mile.
This, of course, presumes that the ghost is displaced horizontally
within the same scan line, and not offset by one vertical trace which
would add 12+ miles plus that 0.4 mile. Where did the extra 2+ miles
come from? Retrace time.

On occasion of poor coax shielding from the cable TV provider, I would
get leakage from strong local TV transmissions that would have ghosts
with huge offsets not only in time, but also picture content. It
would easily compute to 100s of miles path difference, but the lag
time (timed by jump-cuts) proved it was the difference between the
network feed and cable feed time (upwards to a second or two).

So, when we return to this ¼" ghost, for this particular TV set, the
dimension is 0.04 mile or about 200 feet. Sounds like a ringing line.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
So, when we return to this ¼" ghost, for this particular TV set, the
dimension is 0.04 mile or about 200 feet. Sounds like a ringing line.

How does that fit with the state-of-the-art theory that standing-wave
energy doesn't travel end-to-end but instead sorta sloshes from side-
to-side between nodes?
--
73, Cecil http://www.qsl.net/w5dxp


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