G8MNY  > TECH     04.08.25 07:00z 101 Lines 4525 Bytes #122 (0) @ WW
BID : 38079_GB7CIP
Read: GUEST
Subj: VHF ATU Homebrew Design
Path: ED1ZAC<ED1ZAC<GB7CIP
Sent: 250804/0657Z @:GB7CIP.#32.GBR.EURO #:38079 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                             (Updated Aug 05)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This is for use at those times when after setting up a /P (demos) station there
is no time to sort out why the SWR is not quite right!

For VHF wire aerials where the Z is not around 50ohm, see my bul on a 2M
fullwave loft dipole for the wire trombone matching system.

CIRCUIT
This is quite a straight forward pie section design using 2 variable air spaced
capacitors & a select on test coil between them.

 SO239 or              S.O.T.              PL259 or
N Female  o)ÄÄÄÄÄÄÄÄÂÄÄÄCCCÄÄÄÂÄÄÄÄÄÄÄÄ(-  N Male
           ³ 2-30pF ³ 3 Turns ³ 2-30pF ³
           ³       ===       ===       ³
           ÀÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÙ
                   Adjustable Cs

The value of the variable C will need to be smaller than 30pF for higher bands
e.g. 220MHz, & as large as 100pF for 50MHz, if a good impedance matching range
is going to be obtained.

By using a SO239 & PL259 or Female & Male N types for connections, adaptors are
not need.

ATU LAYOUT
                                                         Soldered
 Soldered      C Adj             Coil           C Adj   in Lid Nut
   in ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
      ³³         |~" \.      /'\      /'\         |~" \.   (o)³³ ÚÄÄÄÄÄÄ¿
ÚÄÄÄÄÄ´³   2-30pF³     \     |||      |||         ³     \     ³Ãij      ³
³ SO  ³³_   _____/\_____³   // \\    // \\   _____/\_____³   _³³ ³  PL  ³
³ 239 ³³_].³     \/      ³ ||   ||  ||   || ³     \/      ³.[_³³ ³ 259  ³]
³or N ³³ `\\| Variable  | //    \\ //     \\ | Variable  |//  ³³ ³ or N ³
ÀÄÄÄÄÄ´³   `\\.   C   ./' ||     |||      || `\.   C   .//'   ³Ãij      ³
      ³³(o)  `\\o---o/==='/      \_/       \`===\o---o//'     ³³ ÀÄÄÄÄÄÄÙ
      ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
     Soldered          9cm Long Square U Section
    in Lid Nut

The box needs to be just long enough for all the components. I found that a
square tube of the size of the square end chassis mounting SO230 was OK, but it
was tight for the Caps. That is 9cms x 2.4cm x 2.4cm (3.6"x1"x1")

CONSTRUCTION
1/ Make a chassis PL259 or N plug by mounting a cutting down standard large
   cable plug & soldering it onto a cut down square SO239 base (or brass plate
   1"x1") (see 5).

2/ Cut a sheet of thin brass/copper/tin can 9cm long to form the U section of
   the body.

3/ Bend into a square section U shape using a vice & a 1" former & a hammer.

4/ Punch/drill the fixing holes needed in bottom of U section for the 2
   variable caps, making sure there is room for full vane rotation.

5/ Tin the ends of the U section ready to solder in the 2 connectors. Clean up
   the connectors ready for soldering. If the connector insulation is of the
   easily melted type drop water onto the inner while soldering. Use a hot
   small blow lamp if you don't have a huge electric iron, & solder the 2
   connectors in place. Once done this gives a very strong U section box.

6/ Bolt in the 2 caps making sure the rotary vanes are well earthed. Wire the
   stators up to the external connectors with thick wire.

7/ In the corners, solder in 2 nuts to attach the lid.

8/ Make a lid using some more of the sheet metal 9cm long, but allowing for a
   3mm fold over top & bottom to go out side the U section. Cut & fold it to
   make the shallow U section as before. Punch/drill 2 fixing holes to align up
   with the 2 soldered in nut locations.

CONTROLS
My capacitors had only screwdriver slotted ends, but I glued on 2 small knobs
(e.g. toothpaste tube caps??) & marked them with max & min C positions.

COIL SIZE
This is found on test by setting the 2 Cs to halfway & trying different coils
of fairly thick copper wire between the two caps until the SWR is OK into a
good load. This gives you the best SWR adjustment range centred around 50ohm.

IN USE
An aerial SWR of approximately 3:1 can be matched on 144MHz, 2:1 on 70MHz 1.5:1
on 50MHz with 30pF capacitors.
Another benefit is that the Tx harmonics are reduced through the ATU as well as
giving Rx protection from higher frequencies you may be using such as 432MHz.

Insertion loss should be quite low say 0.2dB, & power handing is 400W PEP SSB
(Contest tested!). But this very much dependent on the thick wire used & air
spacing gap of capacitors, as well as SWR in use. e.g. 250V @ 4A RMS ?


Why Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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