Is this 300 Ohm to 75 Ohm Impedance Matching "Transformer" Defective?
Joe
TV antenna.
The matching transformer looks exactly like the one on this URL:
http://www.solidsignal.com/prod_display.asp?PROD=WMXFMRPT
I have a couple of questions:
I'm not sure if this part is defective. I put a DMM meter on the various
connections and measured the resistance:
Between the inner coax and each of the other three terminals = infinite ohms.
All the other measurements between terminals give 0.4 - 0.3 = 0.1 ohm,
(that is, my probes have 0.3 ohm resistance).
Does this make any sense? I originally checked these resistances thinking
that this "transformer" would be two coils wound on an air core, so that I
would measure a small resistance between the two 300 ohm leads, and a
small resistance between the outer and inner of the coax connector.
I have another DMM that can measure capacitance, but it's not currently in
operation. If recommended, i could try to measure capacitance between the
various terminals.
FWIW the package, but not the part, is marked "Philmore 300-75 OHM
Matching Transformer. No. MT73"
The part itself is marked: UHF/VHF MATCHING TRANSFORMER CHINA
Also, can these matching transformers be used in either direction? That
is, my antenna will get the 300 ohm end, and the TV antenna input will get
the 75 ohm end, is this okay?
Thanks.
--- Joe
Phil Allison
"Joe" <no...@given.now
>I bought a 300 to 75 ohm "matching transformer" to hook up to a home built
> TV antenna.
**Go back to "sci.electronics.basics" - cos your Q has already been
answered.
Part of you Q is answered on the link you posted.
Do pay attention.
..... Phil
Dave Platt
>TV antenna.
#snip#
>I'm not sure if this part is defective. I put a DMM meter on the various
>connections and measured the resistance:
>
>Between the inner coax and each of the other three terminals = infinite ohms.
>
>All the other measurements between terminals give 0.4 - 0.3 = 0.1 ohm,
>(that is, my probes have 0.3 ohm resistance).
>
>Does this make any sense?
It might. There may be a DC-blocking capacitor present in the
connection from the coax center connector to the coil, to ensure that
no damage occurs if you were to plug this sort of balun into a coax
which was being fed DC power for a mast-mounted preamp. The
DC-blocking cap would have a very low impedance at radio frequencies
(it would be effectively "invisible" to RF) but will have such a high
impedance at DC that your meter will read "infinity".
> I originally checked these resistances thinking
>that this "transformer" would be two coils wound on an air core, so that I
>would measure a small resistance between the two 300 ohm leads, and a
>small resistance between the outer and inner of the coax connector.
It's probably wound as a 4:1 "auto-transformer", in which the primary
and secondary windings are actually connected together. This
establishes a current path for DC from the high-impedance side to
ground, and allows any static electricity building up on the antenna
to flow safely to ground. If the primary and secondary windings were
isolated, you could get hundreds or thousands of volts of charge
building up on the secondary side (even in clear dry weather) and the
static charge could arc over to the primary windings and damage the
insulation on the wire.
Tt's very probably wound on a small ferrite core (toroid or two-hole
balun core) rather than with an air core. The high permeability of
ferrite means that fewer turns of wire would be necessary,
Since there are only a few turns of wire, the DC resistance would be
less than an ohm.
>Also, can these matching transformers be used in either direction? That
>is, my antenna will get the 300 ohm end, and the TV antenna input will get
>the 75 ohm end, is this okay?
Yes... you could, for example, connect a 300-ohm balanced antenna to
75-ohm coax with one, and then use a second one to adapt the coax to a
high-impedance balanced antenna input on a TV or stereo.
--
Dave Platt <dpl...@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
Joe
<phila...@tpg.com.au> wrote:
NO ONE has addressed the fact that there is NO apparent continuity from
the center conductor of the coax to ANY of the other terminals.
Do you, Phil, or anyone else know if this indicates that this
"transformer" is defective?
Thanks.
--- Joe
Phil Allison
"Joe the Jerkoff "
>> >I bought a 300 to 75 ohm "matching transformer" to hook up to a home
>> >built
>> > TV antenna.
>>
>>
>> **Go back to "sci.electronics.basics" - cos your Q has already been
>> answered.
>>
>> Part of you Q is answered on the link you posted.
>>
>> Do pay attention.
>>
>
> NO ONE has addressed the fact that there is NO apparent continuity from
> the center conductor of the coax to ANY of the other terminals.
>
> Do you, Phil, or anyone else know if this indicates that this
> "transformer" is defective?
** I suppose it has never occurred to you to TRY the fucking thing ?
If you get weak or no signal - then it is faulty.
..... Phil
Joe
wrote:
Thank you, Dave. It appears that you and I were simutaneously adding to
this thread just a couple of minutes ago.
None of the examples on wikipedia or anywhere else I looked had a
capacitor. So I really wondered if there *might* be one inside the
transformer.
I appreciate the explanation. Even though these things are physically
simple devices, impedance matching of transmission lines sure is currently
somewhat mysterious to me.
=== Joe
DaveM
"Joe" <no...@given.now> wrote in message
news:none-22050...@dialup-4.232.81.73.dial1.losangeles1.level3.net...
I looked at a new antenna transformer; it has DC continuity between all
terminals. That is, any terminal should have continuity to any other terminal.
The resistance will, of course, be very low... in the neighborhood of 0.1 ohms
or so.
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)
Some people are like Slinkies. Not really good for anything, but they bring a
smile to your face when pushed down the stairs.
Michael A. Terrell
Joe wrote:
>
> NO ONE has addressed the fact that there is NO apparent continuity from
> the center conductor of the coax to ANY of the other terminals.
>
> Do you, Phil, or anyone else know if this indicates that this
> "transformer" is defective?
Try it and see. Some have a couple capacitors in them to flatten the
RF response, while others are DC coupled. At one time I tested samples
of every type our company was thinking of using, and there were lots of
variations.
--
http://improve-usenet.org/index.html
Use any search engine other than Google till they stop polluting USENET
with porn and junk commercial SPAM
If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm
Joe
Terrell" <mike.t...@earthlink.net> wrote:
> Joe wrote:
> >
> > NO ONE has addressed the fact that there is NO apparent continuity from
> > the center conductor of the coax to ANY of the other terminals.
> >
> > Do you, Phil, or anyone else know if this indicates that this
> > "transformer" is defective?
>
>
> Try it and see. Some have a couple capacitors in them to flatten the
> RF response, while others are DC coupled. At one time I tested samples
> of every type our company was thinking of using, and there were lots of
> variations.
Michael,
I have finished the homemade indoor version of a UHF antenna and hooked up
the 300 ohm to 75 ohm "transformer". It works.
When I asked the question about the transformer, the antenna had not been
completed, and I really wanted the antenna to be the only variable, thus
the attempt to ensure that the transformer was good.
I built the Gray-Hoverman antenna as shown on
http://www.digitalhome.ca/ota/superantenna/design.htm
My copy of this antenna was really a kluge. Rather than using copper or
aluminum tubing or bare heavy gauge aluminum or copper wire, I used 22 AWG
or smaller insulated copper wire, Scotch taped to a piece or corrugated
cardboard about a quarter inch thick.
The geometry, however, is spot on :)
I have not yet constructed the reflectors which are to be positioned 4" to
the rear of the main antenna plane.
All things considered, it works pretty well, seeing that I'm about 45
miles from Mt. Wilson which is where most(?) of LA's TV transmitters are
located.
A note about Phil Allison:
I had thought about asking about construction materials for the antenna
before I built it, but I thought at least one of the sci.electronics.*
A-Holes would take the opportunity to show their "wit".
Sure enough, old toilet-mouth Phil Allison crawled out of the woodwork.
I sure wish he would at least stay out of sci.electronics.basics. He is
the opposite of a good teacher and takes joy in his attempts to kill
enthusiasm and the joy of learning.
--- Joe
Michael A. Terrell
Joe wrote:
>
> In article <Zoidnc5W2sJehqXV...@earthlink.com>, "Michael A.
> Terrell" <mike.t...@earthlink.net> wrote:
>
> > Joe wrote:
> > >
> > > NO ONE has addressed the fact that there is NO apparent continuity from
> > > the center conductor of the coax to ANY of the other terminals.
> > >
> > > Do you, Phil, or anyone else know if this indicates that this
> > > "transformer" is defective?
> >
> >
> > Try it and see. Some have a couple capacitors in them to flatten the
> > RF response, while others are DC coupled. At one time I tested samples
> > of every type our company was thinking of using, and there were lots of
> > variations.
>
> Michael,
>
> I have finished the homemade indoor version of a UHF antenna and hooked up
> the 300 ohm to 75 ohm "transformer". It works.
>
> When I asked the question about the transformer, the antenna had not been
> completed, and I really wanted the antenna to be the only variable, thus
> the attempt to ensure that the transformer was good.
Having dealt with thousands of the matching transformers the only bad
ones I saw were physically damaged, or hit by lightning. That cable
company bought them 1000 to 5000 at a time, and split the order between
multiple cities. In four years I saw less than a dozen bad ones, and
most of them had been driven over by the subcontractor installers.
> I built the Gray-Hoverman antenna as shown on
>
> http://www.digitalhome.ca/ota/superantenna/design.htm
>
> My copy of this antenna was really a kluge. Rather than using copper or
> aluminum tubing or bare heavy gauge aluminum or copper wire, I used 22 AWG
> or smaller insulated copper wire, Scotch taped to a piece or corrugated
> cardboard about a quarter inch thick.
>
> The geometry, however, is spot on :)
>
> I have not yet constructed the reflectors which are to be positioned 4" to
> the rear of the main antenna plane.
>
> All things considered, it works pretty well, seeing that I'm about 45
> miles from Mt. Wilson which is where most(?) of LA's TV transmitters are
> located.
>
> A note about Phil Allison:
>
> I had thought about asking about construction materials for the antenna
> before I built it, but I thought at least one of the sci.electronics.*
> A-Holes would take the opportunity to show their "wit".
>
> Sure enough, old toilet-mouth Phil Allison crawled out of the woodwork.
>
> I sure wish he would at least stay out of sci.electronics.basics. He is
> the opposite of a good teacher and takes joy in his attempts to kill
> enthusiasm and the joy of learning.
>
> --- Joe
Do you know how to use a kill filter? Rarely does Phil ever say
anything useful. It is so rare that when he does, someone usually
repeats it.