R:190721/0929Z 9661@N7HPX.#BOI.ID.USA.NOAM BPQ6.0.18
R:190721/0920Z 33515@N3HYM.MD.USA.NOAM BPQ6.0.18
R:190721/0926Z @:GB7CIP.#32.GBR.EURO #:13067 [Caterham Surrey GBR]

From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (New May 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

For up to 30MHz this is a rebuild of standard C.B. "ELECT marc" 2 meter SWR bridge. Instead of the original frequency sensitive pick up lines that are OK for 27MHz, it uses a ferrite ring transformer for the current & a resistive tap for the voltage.

FEATURES
5W 50W 500W Power meter ranges.
Separate single SWR meter with full scale Set pot.
RF field strength detector (used separately).

CIRCUIT

Tx Coax 1:26 transformer Ant o)─)===========================================┬──(o
├─┘ ::::::: ┌───┤ │
│ _____())))))_____ 10k 10k │
│ │ 26T │ ├───┘ │
│ ├──68R───┬───68R──┤ │ │
│ │ ├────────)─────┘ │ Field
│ Sch_│_ │ _│_ │ Strength
│ \_/ │ \_/ │ Aerial
│ 1n │ │ │ 1n │ \|/
├────┤├──────────┬┴────────)──────┐ ├───────┤├───┤ |
│ │ │ Foro oRef │ |
│ ┌───┐500W ├────┐+ │ /│\ │ |
│Cal4 220k o === Power │ ├─────────────)──┬─┤<├─┬o
│ ├───o50W │1n Meter │ Set SWR │ │ _│_
│Cal3 100k o<────┴────┘ ┌─┴─┐ SWR<──┬─Meter┐ │ │ /_\
│ ├───┘5W 3k3 │ 220k ===1n │ │ ===1n │
│Cal2 50k Cal1│ 390 log └──────┤ │ │ │
└──────┴─────────────────┴───┴─────┴───────────┴─┴──┴─────┘
Both Meters are 25uA & 1500Ω.
The 10Ks are 1W carbon type.
The 390R is carbon type.
Both 68R are 0.5W carbon type.
Diodes are Schottky or Germanium types.

PRINCIPLE
The 50Ω coax goes through a small HF ferrite 15mm torroidal with a 26 turn secondary, the coax outer is grounded only at one end so 1/26 of the inner current appears across the series 68Rs. The centre of these is connected to a preset voltage attenuator so that with a 50R load the reverse signal cancels.

The forward power is measured via 3 series presets into a separate meter.

For SWR the Forward & Revers signals are measured via the adjustable pot to the SWR meter.

For the field strength a small aerial is screwed in & the disconnected SWR meter can now show local RF fields. (e.g. along your coax outer!)

CALIBRATION (done in order as they affect each other!)
Cal1. set Reverse SWR null reading on a good load
Cal2. set 5W full scale deflection
CaL3. set 50W full scale deflection
Cal4. set 500W full scale deflection.

N.B. the meters have square law power scales & an SWR scale.


See also my Tech buls on "Drake WH7 QRO HF SWR Bridge",
"PEP Meter modification", "QRP SWR Bridge","QRO 1kW HF Metered Dummy Load",
"HF ATU & SWR Bridge VC300LP/QT-1" & "Meter Damping & Speed Up".


Why don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP


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