| |
G8MNY > TECH 15.11.20 10:41l 119 Lines 4768 Bytes #999 (0) @ WW
BID : 12696_GB7CIP
Read: GUEST
Subj: HF Balun & Marconi T switch
Path: IZ3LSV<I3XTY<I0OJJ<GB7CIP
Sent: 201115/0932Z @:GB7CIP.#32.GBR.EURO #:12696 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (New Jun 15)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
I tested out a junk sale buy "a transformer 1:1 balun with Marconi T switch",
but it was useless. The large ferrite/iron dust core has 10 turns of 6 strand
ribbon cable on it, all dipped in wax.
Basic 1:1 Transformer Balun (works OK with high Z RF earths!)
Core handles only 50% of the power
Coax o)ÄÄÄ¿
50ê ³ ')||
Unbalanced ³ )||
³ ÃÄÄÄÄÄÄÄ
³ ')||
Switch ³ )|| 50ê
Break \oÄ´|| Balanced
for T ')||
)||
ÀÄÄÄÄÄÄÄ
The transformer losses/SWR were bad & the Marconi T mode, just disconnects the
earth, & so it did not drive the dipole equally & left some of the inductance
in series.
G8MNY RE-DESIGN
So I reworked it with a much better ferrite core to handle 400W 160m-10m.
Added a double pole changeover switch for proper Marconi T, AERIAL USE switch.
And a switch for a low loss floating Choke Balun, MODE switch.
As well as 3mm aerial spark gaps at the terninals.
Coax o)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
50ê ³ 1')||
Unbalanced ³ )||
³ Transformer/ ÃÄÄÄÄÂÄÄÄÄÄÄÂ>A 50ê Aerial Posts
³ Choke MODE 2')|| ³ Spark³ Transformer Balun,
³ o\______)|| ³ Gaps³ or Choke Balun feed,
ÃÄÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÄ¿|| ³ Ú< or Teed for MF
³ ³ 3')|| ³T ³ ³ Marconi T Confuguration.
³ Dipole o )|| o ÚÄÄ)ÄÁ>B
³ / oÄÄÄÄÄÄÁÄÄÄÄo/ ³
³ ³ T Dipole ³
ÃÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
Earth_³_ AERIAL CONFIG SWITCH
post ///
HOW IT WORKS
For Transformer Balun Mode, the 3 windings are all in series, with windings 1 &
2 seeing the unbalanced coax & windings 2 & 3 seeing the balanced aerial. The
aerial balance is forced to be exactly half voltage on each side of earth due
to the auto-transformer action. As the source & load see the same number of
turns the Z is unaffected.
In Choke Balun Mode, the voltage equalising winding (2) is disconnected from
earth. The aerial balance is fee to float with respect to earth limited only by
the choke inductance. If the 2 winding lengths are short & kept parrallel
(bifillia) the Z should stay close to 50ê.
In Marconi T configuration, both legs of the aerial are fed directly (from ATU)
The voltage equalising winding (2) is disconnected, & the inductance of winding
(1) is shorted out as winding (3) is shorted.
CONSTRUCTION
All leads kept very short & balanced output leads twisted. Flat 3 wires
(Trifillia) winding can is used for 1 2 & 3, or a 50ê coax can be used for 1 &
3, with 2 just being a wire, this will give the best match & lowest loss in
Choke Mode. Care must be taken to get the 3 winding starts (') wired correctly.
Aerial posts
ÚÄ¿ ÚÄ¿
ÃÄ´ ÃÄ´
EarthÚÄÄÄÅÄÅÄÄÅÄÅÄÄÄ¿
post³ T_ _T ³
ÚÄÂÅ_______Ý ³
ÀÄÁÅ _-ÄÄ-_ ÜÛÛ³ Dipole
³ / \ÜÛÛÝ/ AERIAL TYPE
³ | __ ÜÛÛÝ T
Choke ³ÛÛÜ / \ |ßß³
MODE \ÞÛÛÜ \__/ Þ ³
Transformer ÞÛÛ| | ³ Shack
³ßß \_ _/ ³ Coax PL359
³ Ä--Ä ==============[-
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
TESTING
Put a non inductive 50R between the Aerial Posts A & B, the SWR should be
perfect on all bands in Choke Mode.
In Transformer Mode, SWR should also be good across all bands, but may be
affected by too low inductance L (too few turns) @ LF & too many turns for HF,
where the total winding length of all 3 windings is > 5% of the wavelength or
wires to far apart etc.
Balance tests of earthing A & then B, should show equally high SWRs in
Transformer Mode & no SWR change in Choke Mode of the L is high enough @ LF.
In Marconi Mode only a 50R between either Aerial Posts A & B to Earth Post will
give a good SWR.
If a QRO power source & a power balanced dummy load is available, then core
heating & saturation tests can be done. SWR should not change with full power
@ LF or heat up excesively @ HF. Do not throw the switches at full power!
IN USE
For external use, greasing up all terminals, switches & connectors is
recomended, but water proof boxes usually fail or sweat. A small breathing hole
at the lowest point will let any water out.
The Choke fed mode can give better feeder noise isolation, if the aerial is not
perfectly balanced in practice.
See my tech bul "12 Uses of HF Trifilair L" for full Ferrite ring suitability
test.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
Read previous mail | Read next mail
| |
