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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?

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The PAoNHC 2mtrs duplexfilter made from copperclad, as used with PI3RTD (7k).

Configuration:

The two extra cavities attenuate very strong, in-band and near-band signals to aviod:

Properties:

Pre- and final adjustments possible with:


Adjustments:

Specifications:

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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 (11k).
Narrow-band performance of the PA0NHC duplexfilter.

Wide-band performance of the PA0NHC duplexfilter (6k).
Wide-band performance of the PA0NHC duplexfilter.



Click here for the description
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:

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!

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