On Jan 18, 10:32*pm, Jeff Liebermann wrote:
On Sat, 17 Jan 2009 12:44:05 -0800 (PST), JIMMIE

wrote:
I was considering using a TV type 4:1 balun on 2 meters. Does anyone
know how much power these can handle? I remember a guy doing this when
I was a teenager(long ago) but I think the baluns were built a little
stouter back then than they are now.

May I suggest using a coaxial cable 4:1 balun instead of ferrite core
type. *A 1/2 wave electrical length of coax cable, some soldering, and
you're done. *For just 2 meters, the bandwidth is just fine and the
loss is no more than that of the coax cable. *For low power, small
diameter RG-174 type coax is fine. *I use 4:1 coax baluns for matching
300 ohm folded dipole antennas:
http://users.belgacom.net/hamradio/schemas/14balun.gif

The common TV balun has two xfomers inside, which are sometimes
combined on a single core. *See schematics at:
http://members.tripod.com/rclindia/trans.html
http://www.electronics-tutorials.com/basics/baluns.htm

However, if size is an issue, I guess the coax balun is a bit large.
For just 2 meters, the ferrite core is not necessary. *Just find a
block of PTFE (Teflon) or plastic, and drill it to resemble the TV
balun core, use 2 PTFE toroid cores (if you can find them), or
simulate a torroid with a stack of nylon washers. *Then wind the 2
pairs of 2 wires through the holes and bring out the connections.

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

I was rummaging through my mobil junk er toolbox and came across an
old package of air core TV baluns. These date back to the day when
TVs were usually fed with twin lead. Tuesday I may be able to get to
some test equipment to check out the losses on those. Gee getting to
spend a 3day weekend on the road is such fun.

Jimmie

On Jan 18, 10:32*pm, Jeff Liebermann wrote:
On Sat, 17 Jan 2009 12:44:05 -0800 (PST), JIMMIE

wrote:
I was considering using a TV type 4:1 balun on 2 meters. Does anyone
know how much power these can handle? I remember a guy doing this when
I was a teenager(long ago) but I think the baluns were built a little
stouter back then than they are now.

May I suggest using a coaxial cable 4:1 balun instead of ferrite core
type. *A 1/2 wave electrical length of coax cable, some soldering, and
you're done. *For just 2 meters, the bandwidth is just fine and the
loss is no more than that of the coax cable. *For low power, small
diameter RG-174 type coax is fine. *I use 4:1 coax baluns for matching
300 ohm folded dipole antennas:
http://users.belgacom.net/hamradio/schemas/14balun.gif

The common TV balun has two xfomers inside, which are sometimes
combined on a single core. *See schematics at:
http://members.tripod.com/rclindia/trans.html
http://www.electronics-tutorials.com/basics/baluns.htm

However, if size is an issue, I guess the coax balun is a bit large.
For just 2 meters, the ferrite core is not necessary. *Just find a
block of PTFE (Teflon) or plastic, and drill it to resemble the TV
balun core, use 2 PTFE toroid cores (if you can find them), or
simulate a torroid with a stack of nylon washers. *Then wind the 2
pairs of 2 wires through the holes and bring out the connections.

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

Jeff, Apparently TV matching transformers used to be air core and
their use was quite common on 6 and 2 meter antennas.I knew the ones
built into the old tuners were but I hadnt thought the old line
transformers also being air core.They were per some of the locals on
2m repeater. If they aare the ones I am thinking of(big fat ones) I
have a few at home.

Jimmie

On Mon, 19 Jan 2009 08:10:07 -0800 (PST), JIMMIE
wrote:

Jeff, Apparently TV matching transformers used to be air core and
their use was quite common on 6 and 2 meter antennas.

Not on any 2m or 6m antenna I've seen or build. Most of what I've
seen used T-match, hair-pin match, gamma match, or something similar.
For the vertical collinears, I used a tapped coil. If I wanted lots
of bandwidth, perhaps a ferrite core might be useful, but for single
VHF band antennas, it's a lossy waste.

I knew the ones
built into the old tuners were but I hadnt thought the old line
transformers also being air core.

The over-simplified idea behind the ferrite (or powdered iron for HF)
cores is to increase the inductance of the windings so the xformer
works at lower frequencies. At VHF and UHF frequencies, the core
mostly disappears and you're relying totally on the coupling between
the bifilar windings.

They were per some of the locals on
2m repeater. If they aare the ones I am thinking of(big fat ones) I
have a few at home.


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

On Mon, 19 Jan 2009 09:43:27 -0800, Jeff Liebermann
wrote:

The over-simplified idea behind the ferrite (or powdered iron for HF)
cores is to increase the inductance of the windings so the xformer
works at lower frequencies. At VHF and UHF frequencies, the core
mostly disappears and you're relying totally on the coupling between
the bifilar windings.

Actually that is not the mechanism at all, simplified or complexified.

Ferrite compounds for BalUn application principally add resistance to
the common mode path and have (as a design goal) absolutely no
magnetic influence whatever. The increase of inductance is haphazard
at best, and some ferrite compounds actually increase the capacitive
reactance. As such, ferrites are not principally employed in building
inductors. Further, iron cores don't serve much purpose for choking
which is the principle requirement for BalUn input to output
isolation.

If you want to build an iron core, magnetic linkage transformer (AKA
audio or power transformer), this is a very different beast from a
BalUn, and a miserable replacement.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On Mon, 19 Jan 2009 09:43:27 -0800, Jeff Liebermann
wrote:

The over-simplified idea behind the ferrite (or powdered iron for HF)
cores is to increase the inductance of the windings so the xformer
works at lower frequencies. At VHF and UHF frequencies, the core
mostly disappears and you're relying totally on the coupling between
the bifilar windings.

Actually that is not the mechanism at all, simplified or complexified.

Ferrite compounds for BalUn application principally add resistance to
the common mode path and have (as a design goal) absolutely no
magnetic influence whatever. The increase of inductance is haphazard
at best, and some ferrite compounds actually increase the capacitive
reactance. As such, ferrites are not principally employed in building
inductors. Further, iron cores don't serve much purpose for choking
which is the principle requirement for BalUn input to output
isolation.

If you want to build an iron core, magnetic linkage transformer (AKA
audio or power transformer), this is a very different beast from a
BalUn, and a miserable replacement.

73's
Richard Clark, KB7QHC

When designing any transformer, be it audio, power, or RF, you usually
want to accomplish three things:

1. Maximize the winding impedance.
2. Minimize the leakage impedance. This is another way of saying you
want to maximize the coupling between the windings.
3. Realize an acceptably low loss.

At low frequencies, most cores primarily provide inductance, so most
literature dealing with transformers speaks of "inductance" rather than
"impedance" when discussing the basic goals. But low frequency
transformers could be made and would function just fine using resistive
core materials if they were available. The third goal can be achieved
with resistive materials as long as the first and second goals are
accomplished well enough. That is, if the winding impedance is high
enough, the magnetizing current will be low, so I^2*R loss will be low.
And if coupling is good, increasing the current in one winding won't
appreciably increase the core flux and incur extra loss.

But at RF, as Richard says, many ferrite cores are more resistive than
inductive. Not all are -- there are high frequency ferrites that remain
inductive at very high frequencies. However, for the HF range and above,
the best choice is usually "low frequency" ferrites which are primarily
resistive in that frequency range. They're best for two reasons --
first, they provide much more impedance per turn squared than high
frequency ferrites, making the first two goals much easier to achieve;
and second, there are no resonance effects. An inductive winding will
resonate with stray capacitance at some frequency. The impedance drops
above that frequency, limiting the upper range of the transformer. The
winding and leakage inductances can also resonate with external
capacitance and cause ringing in some applications. A properly designed
transformer using an essentially resistive core can operate very well
over several decades of frequency.

For most RF applications, it's not hard to design a transformer or balun
that will have well under 1 dB of loss, which is negligible again in
most applications. But if high power is involved, even a fraction of a
dB might represent several or may watts of power, resulting in
intolerable core heating. In those applications, lower loss, i.e., more
inductive and less resistive, cores have to be used, often resulting in
more difficult design and/or reduced performance in other respects.

Resistive ferrites are also often good choices for RF chokes for the
same reasons that they make good transformers.

High-Q inductors and tuned transformers are a different critter, though.
For those applications you want to use very low loss, that is, inductive
and not resistive, core materials. This often leads to confusion, since
ferrite manufacturers have traditionally specified a "recommended"
frequency range for various materials which refers to the best range for
these applications. The best material for an RF transformer is usually
one with a "recommended" (for high-Q inductor applications) frequency
range well below the transformer operating frequency.

Roy Lewallen, W7EL

Jimmie,
I have tried and used these tv baluns so I could run 300-ohm cable to my
antenna. BUT I only ran about 5 watts.
I used one on both ends of a 100-ft run of 300-ohm cable ( so as to reduce
losses) . (And I used it on 10 meters, not 2 meters)
I later wound my own impedence-matching 4-to-1 baluns to handle a full
100 watts.
You're on the right track of having fun with transmission lines, impedence
matching, and antenna building - keep it up.
W4PMJ - hal

"JIMMIE" wrote in message
...
I was considering using a TV type 4:1 balun on 2 meters. Does anyone
know how much power these can handle? I remember a guy doing this when
I was a teenager(long ago) but I think the baluns were built a little
stouter back then than they are now.

Jimmie