In a message dated 7/1/01 11:38:12 AM Pacific Daylight Time, realex@flash.net
writes:
<< MIke,
Since you did not specify narrow or broad band, Ill take the easy route and
say make a choke balun out of a length of 75 ohm coax that is 1/4 wavelength
long at the desired frequency. Connect the 100 ohm load to the end with the
choke
and a 50 ohm line to the other end. Dont forget to account for velocity
factor of
the 75 ohm line.
73, Bob, W5AH
-----Original Message-----
I need a drawing for a 2:1 balun. Please respond direct to me.
73, Mike KF8OD
>>
The suggested balun will work after fashion as the so called Choke coils
aren't as effective as many claim. Attach a Palomar RF Current Meter below
the choke and it shows there is still RF Spill Over below the choke--perhaps
less.
The "1/4 Wave Bazooka" construction will give literally a 100 percent choke
affect and increase the bandwidth substantially. The shorted 1/4 wave stub
construction creates the opposite reactance the antenna DE does as it's
equivalent to an open 1/4 wave stub. It works on odd harmonics also. Adding
the 1/4 wave 75 ohm stub will give the desired 2:1 step up to 100 ohms. Be
sure to seal the coax joint properly and the feedpoint at the top. I'd use a
mail and female Type N connectors and tape them. A 92 ohm stub within the
Bazooka will allow a match to a 169 ohm load +/- on odd harmonics also. If
you added a 300 ohm 1/4 wave stub you could match a 532 ohms load +/- and
higher loads with higher Zo open wire lines.
In the "Old Days" Mfgs like Collins used to add a diagram of the "Collins
Bazooka" in their Transmitter Instruction Manuals so that their transmitters
would work better. It would also keep the RF Spill Over out of the first
audio mike stage and "RF Bites" off the mike. The older Antenna Handbooks
showed the construction of the Open and Sleeved Bazooka. The open Bazooka
has to be keep away from the tower and other metal objects. The 2nd leg can
be made form old coax as only the shield is used.
In the Proceedings of the IRE there was a "so called improvement" of the
Bazooka using an open 1/4 wave stub in the 2nd arm connected in series.
Unfortunately the Editors of the IRE Mag didn't check it theoretically or in
a working model and it "decreased the bandwidth"--in actual practice. I sent
the editor of IRE a 2M sample of the original and their model and they didn't
answer or print a retraction. It's awfully tough never being wrong.
Many 144/450 MHz quads are sold without baluns and ranks up in the "K7GCO Top
5 All Time Dumb Dumbs" of Antenna Design. The vertically polarized RF Spill
Over excites the coax shield and mast which lowers the F/B even of a
horizontal beam. It's even worse when a single antenna is vertically
polarized on the mast for this and another reason. It's difficult enough
getting a clean vertically polarized pattern even when a beam is supported
away from the mast. On the lower gain 2M beams like a 3 element vertically
polarized yagi which have fairly close spacing at 1/2 WL or slightly wider
spacing for the optimum horizontal pattern, the supporting horizontal boom
gets into the RF act even with no RF Spill Over on to the coax and becomes a
part of the antenna horizontally polarized--in Eznec and in actual practice.
This affect is minimized with a quad DE, insulated yagi boom and/or the
connecting horizontal boom or the yagi DE insulated from a metal boom. More
on this later. In the year 2001 and with all the hi-tech equipment of today,
there are still some very simple RF Design Problems (fully measurable)
evading John Q Ham today--even when repeatedly exposed. Use a Bazooka on
these quads and also get additional bandwidth. The 100 ohm Bazooka is made
to order for quads and is easy to make.
The Mfgs sell Rigs to John Q Ham. They are now at the "Total Mercy of the
average and mostly non-technical Hams ability to install the Proper Antenna."
It can take 10 years or more for John Q Ham to get up to Antenna Speed
Technically and Financially so that this rig can poke out a commanding signal
in the proper antennas. It's as if a new rig only works better in its later
years. I call this a "Very Poor Marketing Plan"--the very worst I have even
seen. Likewise in Bench Rest Rifle Shooting it takes an average of 10 years
to be competitive even with the same gun. I didn't know that and was a
winner from day one and saved a lot of time, money, retained my interest and
ended up spending a lot of money on guns that performed very well indeed.
New cars and many other products like a TV perform fully from day one or they
would be immediately returned. They often decrease in performance, power or
picture quality with time which is expected.
I have a "Great Marketing Plan" that would increase business immediately and
save John Q Ham a lot of money and time. Reduce John Q Hams "Get Up To Speed
Big Signal Time". This is a "2 step plan."
1.) When they sell you a "Lemon Rig"--don't make the customer "eat it."
Keeping it in circulation is very damaging to future sales of all their rigs
but they don't what it either for strange reasons? Even dealers are doing
this now as they claim "the customers still come back". They don't count
very well. Sales statistics actually show this a very dumb practice. Other
"Professions" are doing this to everyone as they all do it--you are in a
captive market. If you treat your customer fairly--you may have visitors
after dark. Shades of the "Untouchables" even today.
2.) Provide John Q Ham with Get Up To Speed Tower, Rotator, Feedline (like
Collins) and Antenna Manuals/Info/Videos of "Proven Antennas and Systems."
Conduct Antenna Seminars at Conventions & Clubs. Chip Margelli of Yaesu
actually did this. A good close up Video of a complete antenna system has
great sales power. Have a library of such tapes along with parts lists and
costs. Call it "Brand XXX Secret Antenna Files" for their customers--and
other Spies. There would be an "Underground Market" for this info. Do you
remember how long it took you to get up to speed? Rigs connected to poor
antennas tend to gather dust. "Long Live the Bazooka with Modifications."
"If I sold a very hi-tech product that required very hi-tech antennas to
fully support it, I sure as hell wouldn't leave it up to mostly non-technical
customers to properly support my product and my retirement". (Old K7GCO
Axiom) I'd help them all I could. Lets see who picks up on this Sure Fire
Marketing Plan? Another Sun Spot Cycle Low is a just few years off. How
many Mfgs and Radio Stores will survive it--and computers with the Government
Monkey mostly off Gates back? We shall see! Of course they will need
someone to ''Properly Prepare" this material. I could suggest someone. Even
certain material from Technical Reflectors could be reprinted by the Mfgs.
K7GCO
List Sponsor: Are you thinking about installing a tower this summer? Call us
for information on our fabulous Trylon Titan self-supporting towers - up to
96-feet for less than $2000! at 888-833-3104 <A
HREF="http://www.ChampionRadio.com";>
www.ChampionRadio.com</A>
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|