PA0NHC cheap easy 2m repeater duplexfilter.
Low budget easy homebuild and adjusted 8 cavity coupler.
This page is modified 17-4-98.
I am the builder of the two regional amateurrepeaters (2m PI3RTD / 70cm PI2RTD) in Rotterdam. For the 2mtrs-relay someone donated a 4-cavity 2-transmitter-coupler, originally operating on 155/160 MHz. After some measurements and experiments they seemed to be unusable, due to the fact that to much difficult mechanical modifications where nessacery to modify them as a duplexfilter (notchfilter) for 145 MHz, thereby keeping the good mechanical stability, and only 4 cavities where avalable (not enough selectivity).
So, could i build a cavityfilter myself and tune it, without the use of expensive materials, machines and measuring-instruments?
HOME
Configuration:
- 2 x 4 cavities:
- 2 x 3 cavities for deep notch
- and 2 x 1 extra cavity for improved selectivity
The two extra cavities attenuate very strong, in-band and near-band signals to aviod:
- Receiver-overload and -intermodulation
- Desensibilisation by transmitter-sidebandnoise on Frx and the mixingproduct from 2*Ftx and the sidebandnoise on Ftx+(Ftx-Frx)
- Backdoor-modulation in transmitter final stage (strong off-freq. sigs received in the antenna)
Properties:
- Cheap construction with simple tools:
- No turning on a lathe
- No silverplating (yet stable in time !)
- Tin-soldering with a simple 50 Watts thermostate solder-iron
- No expensive materials:
- 22mm copper waterpipe
- Copperclad
- Tinsolder
- Two component glue.
- Blank spray paint
- CRC (2-26 / 3-36 / 5-56)
Pre- and final adjustments possible with:
- VSWR-meter and dummyload
- A battery-powered 2-mtrs portable transceiver acting as a signalgenerator
- Home made attennuators and
- Some sort of good sheelded 2mtrs-receiver as indicator (a bandscope for 2mtrs is very handy)
Adjustments:
- Simple adjustment of in- and output-coupling only once during construction of each individual cavity
- COURSE- and FINE frequency-adjustment
Specifications:
- 2 mtr band (145 MHz).
- Dimensions: abt. 70 x 35 x 18 cm.
- Isolation between RX and TX - ports: 90 dB or more.
- Insertionloss: abt. 4 dB.
- VSWR: lower then 1:1,5
- Good narrow- and wide-band selectivity

Here are the frequency-characteristcs of the filter.
The performance is plotted on the safe side. Lateron, measured with professional equipment, the notch-deepnes showed to be better then -100dB.

Narrow-band performance of the PA0NHC duplexfilter.

Wide-band performance of the PA0NHC duplexfilter.

Click on the picture to read the description of the construction of the pa0nhc 2mtrs repeater duplex filter.
Facts and questions about some duplexfilter-constructions.
A duplexfilter is neccesary, in order to connect a repeatertransmitter and -receiver on the same antenna during "full-duplex" operation (what the receiver is hearing, the transmitter sends at the same time through the same antenna at the same frequency-band). Professional duplexfilters are expensive, and sometimes not good enough for the high-demanding amateur-service. Homeconstruction is often expensive and difficult, due to the use of thick copperpipe and -plate, silverplating, the need of machining it on a lathe, and the use of tracking-generator and spectrumanalyser for adjustment.
In amateurliterature i found a few descriptions for construction of duplexfilters from copperpipe, aluminium sheet or copperclad. I studied a few of these designes, and did build a few testcavities. But i found the descriptions incomplete, the construction expensive or difficult, or the construction mechanically instable.
After the first experiments a lot of questions arose:
- How to get a stable constructed cavity, made from copperclad?
- How to simplify the construction of the tuning mechanism?
- How to get the right separation between the notchfrequency and the bandpassfrequency for the inductive-bypassed cavities?
- What amount of mismatch (VSWR) is tolerable?
- What is the right amount of in- and outputcoupling for each cavity and the influence on:
- Insertionloss
- Notch-deepness
- The interaction between notch- and bandpassfrequencies
- Unwanted interaction between neighbouring cavities?
- What are the correct cable lengths for interconnection of cavities, and coupling of the outputs to the antenna?
- How to adjust the individual cavities and/or the whole combination, without the use of expensive or unavalable laboratory-equipment?
- How to measure the performance, without the use of expensive or unavalable laboratory-equipment?
Therefore i made a full 1/4 wavelength cavity, to test my own construction idees. During experiments devellopping the nessesary constructiondetails, with in the back of my mind: KISS (Keep It Stupid Simple).
The resulting construction could be the answer for those amateurs, who cant buy a professional duplexfilter, but still want a very good seperation between transmitter and receiver for their repeater operating on ONE antenna!