G8MNY > TECH 31.03.25 09:15z 93 Lines 4280 Bytes #100 (0) @ WW BID : 29243_GB7CIP Read: GUEST Subj: RF Directional Coupler Path: ED1ZAC<ED1ZAC<GB7CIP Sent: 250331/0909Z @:GB7CIP.#32.GBR.EURO #:29243 [Caterham Surrey GBR] From: G8MNY@GB7CIP.#32.GBR.EURO To : TECH@WW By G8MNY (Updated Jul 19) (8 Bit ASCII graphics use code page 437 or 850, Terminal Font) This is an item used for VHF broadcast transmitter monitoring. It provides a low level calibrated RF sniff point in the aerial coax for Ofcom (UK regulator) spectrum analyser etc. without turning the Tx off. It is not generally used by other RF users! ŚÄÄÄÄÄÄÄÄæ ³Spectrum³ ³Analyser³ ĄÄÄÄĀÄÄÄÄŁ Aerial .ś'ś. System To Ant To Tx \³/ ŚÄĮÄÄÄÄÄÄÄĮÄæ ŚÄÄÄæ ĄÄÄÄÄÄÄ´DirectionalĆÄÄÄ´Tx ³ ³ Coupler ³ ĄÄÄÄŁ ĄÄÄÄÄÄÄÄÄÄÄÄŁ CIRCUIT This is quite a straight forward 3 line directional coupler as used in some VHF SWR bridges, but here all the lines are very accurate 50ohm transmission lines. To Aerial Loss BNC dB ŚÄ(o)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄæ -16 ´ _.- Aerial ³ ĄÜÜÜÜÜÜÜÜÜÜÜÜÜÜ50R´ Tx -22 ´ _.-' N Female o)ÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ(o N Male -28 ´ _.-' ³ .ÜÜÜÜÜÜÜÜÜÜÜÜÜÜ. ³ -34 ´ _.-' Ć50R ³ ³ -40 ´ _.-' ĄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ(o)ÄŁ -46 ´ _.-' BNC -52 ´-' To Tx -58 ÅÄÄÄĀÄÄÄĀÄÄÄĀÄÄÄĀÄÄÄĀÄÄÄĀ 12 25 50 100 200 400 800MHz Such couplers do not have a flat response, they typically have a 6dB per Octave rise until they are near a resonance or some other losses affect them. Ofcom said some spectrum analysers have calibration offset parameters that can allow for this? (I have never seen one) A HOMEMADE ONE Simpler one I made up was from PCB.. To Aerial To Tx BNC BNC ŚÄ(o)ÄÄÄÄÄÄÄÄÄ(o)Äæ Aerial ³ ŽÜÜÜÜÜÜÜÜÜÜÜŻ ³ Tx N Female o)ÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ(o N Male ³ ³ ĄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄŁ The 'N' connectors were bolted to 1" x1" PCB plates, the whole double sided PCB box soldered up around them & the double sided PCB lines inside that. The 50ohm double sided PCB lines were salvaged from an old VHF SWR detector bridge. But to make a really good accurate match, it needed solder added to them to get a deep SWR null >30dB, on a good load. For this single pick up line design a 50R BNC terminator plug is used in the unused port. IN USE There are accuracy limits to all test gear, here if the transmission system is not 50ohm at the frequency of interest & the spectrum analyser results can be meaningless! This is generally the case off frequency on a resonant aerial. Actual power measurement is possible provided all the patch lead losses are known & the match is good. However the directional coupler does have the ability to show up which of any RF mixing signals are internal & external to the Tx! So nearby corroded metal joints, or other nearby transmitter PAs acting as a mixer etc, can easily be identified as "QRM not in the local Tx". So it is very useful when tracking down the real cause of QRM. ³ Local² ³ ³ ³ Tx² ³ Local ³ Local ³ ² ³ Tx² ³ Tx² ³ ² ³ ² Other ³ ² Other ³ ² Other ³ ² ŽTx ³ ² ŽTx ³ ² Tx ³ ² Ž ³ Mix ² Ž Mix ³ ² Ž ³ ² Ž ³ ³ ² Ž ³ ³ Mix ² Ž Mix ³ ² Ž ³ ³ ² Ž ³ ³ ³ ² Ž ³ ³ Mix ² Ž Mix ³ ³ ² Ž ³ ĄÄÄĮÄÄÄÄÄÄÄÄÄÄÄÄÄĮÄÄÄÄ ĄÄÄĮÄÄÄÄÄÄÄÄÄÄÄÄÄĮÄÄÄÄ ĄÄÄĮÄÄÄÄÄÄÄÄÄÄÄÄÄĮÄÄÄÄ Tx Port Spectrum Aerial Port Spectrum Aerial Port Spectrum Showing Local Tx Mixing Showing External Mixing (mix smaller than Tx Port) (mix bigger than Tx Port) See my tech bul on "FM Stereo Radio Principles", & "A Versatile Pulse Tester" Why Don't U send an interesting bul? 73 de John G8MNY @ GB7CIP
Read previous mail | Read next mail