EMRFD Message Archive 2819

Message Date From Subject
2819 2009-03-29 22:21:05 Ashhar Farhan A question on transmatch
take a look at this :

for those too lazy to open a pdf, i want your opinion on the following:

i am planning to build a balanced line tuner to feed by open wire
antenna transmission line. most of the balanced line tuners are
designed with a transformer that is grounded on the center tap of the
secondary (giving it the balance). the primary is grounded at one end
and fed via a serial variable capacitor from the transmitter. the
serial variable capacitor is used to cancel out the reactive load.

as i set out build this, i am confronted with having to mount the
serial capacitor with both ends 'hot'. this is a challenge of modest
sorts. i am quite clumsy with my metal work, and i would prefer to be
able to simply mount it over the metal chassis.

so, the question is, can i take out the serial variable capacitor
that's between the transmitter output and put it back between the
ground and the other end of the transformer? in this configuration the
hot end of the transmitter output directly connects to the
transformer's one end of the primary and the other end of the primary
goes to the ground via the variable capacitor.

purely from a network perspective (looking from the transmitter end) i
don't see any problem with this (the two reactances are in serial
order, the precise ordering should not matter). on the other hand, the
secondary should only bother with the EMF setup regardless of how the
primary 'floats'. and the primary will have to same amount of current
in any case.

if my reasoning is true, then why have we been floating the serial
input capacitors all this time? is there a lore to it? am i missing
something here?

- farhan
2823 2009-03-30 07:18:42 victorkoren Re: A question on transmatch
Your proposition could be OK if the inductor would'nt have the center tap.
In the original tuner connections, the series capacitor and the inductance to ground act as an inverted L matching circuit, transforming the 50 Ohm from the transmitter to a different impedance across the inductor.
In your connection the transmitter will be connected directly across half of the inductor, so no impedance transformati
2824 2009-03-30 07:43:18 Ashhar Farhan Re: A question on transmatch
the center tap is on the secondary, not the primary.

- farhan

2827 2009-03-30 09:46:16 Stephen Wandling Re: A question on transmatch

L1, with the centre tap, is considered to be the primary in this
circuit. And, both C2 and C3 have to be floating, i.e., not grounded on
either side. If you are using the polycaps as specified, the tapped
mounting holes are isolated from the capacitor, so you can mount them to
a metal panel as long as the tuning shaft does not touch the panel.
When mounting the polycap be sure your mounting screws do not pass too
far into the capacitor, as they will then come in contact with the
capacitor plates.


Ashhar Farhan wrote:
> the center tap is on the secondary, not the primary.
> - farhan
2829 2009-03-30 13:01:40 victorkoren Re: A question on transmatch
Is the schematic similar to the one at the PDF file you attached (Figure 35) because it seems you are refering to another schematic.
Victor 4Z4ME

2832 2009-03-31 07:39:52 Bill Noyce Re: A question on transmatch
As Victor mentioned, the traditional Z-match, including the BLT, has a
grounded tap on the transformer primary (the port where the
transmitter connects). But in looking around, I ran across this
where the primary is grounded at one end, and the variable capacitors
are grounded too. The author doesn't claim that it has the same wide
matching range as a traditional Z-match, though it's not immediately
obvious to me why not. Most of his explanation deals with tuning a
coax-fed antenna, but he also shows how to add a link for balanced

For my own Z-match, I basically followed the W6JJZ article here:
I used a pair of 2x350pF capacitors, and mounted them to a base-plate
sheet of acrylic (Plexiglas?), then mounted the acrylic to my aluminum
case. (If I were doing it again, I would just sandwich the acrylic
between the capacitors and the case, drill through both case and
acrylic, and use nylon screws.) To insulate each capacitor's shaft, I
attached a 1/4" OD nylon spacer as a shaft extension. To attach the
extension, I used a short 1/4" ID nylon spacer, tapped for two screws:
one for the capacitor shaft, and one for the extension.

In short, don't feel intimidated by the job of insulating the
capacitors. I certainly don't consider myself a metal-working wizard
-- this was all done with simple hand tools. This tuner is now a
workhorse in my station.
-- Bill, AB1AV
2836 2009-04-01 09:55:24 Ashhar Farhan Re: A question on transmatch
thanks all, let me finish the ladder line this weekend, i will then
get onto mounting the capacitors.

- farhan