13 August 2007 NW2S de S.I. Scott,
Charlie, N5TD, offers you good advise with his recommendations for eliminating your "Mode-J" desense problem.
There are several possible sources of the overload trouble, viz: -
1. Third harmonic output from your 2M Tx.
2. Brute force (2M) overload of your 70cm Rx system, particularly a low noise pre-amplifier. This will generate 70cm energy even from a 100% clean 2M signal.
3. Third harmonic generation (of the 2M signal) by a "dirty" joint, which acts like a diode, in the antenna system.
The solutions that we offer are: -
1. Our 2M Lowpass filter, model MMf200-7. This is a 7-section, lumped constant, lowpass LC filter. We guarantee at least 40dB suppression at 435MHz, reference the 2M response. Typically you will get over 50dB suppression. The insertion loss (actually reflection loss) at 2m is typically 0.5dB. You install this to the 2M transmitter's output coax connector.
2. Our 70cm, low loss, Bandpass filter, model PSf432. This is a 3-section, 0.1db ripple, Interdigital, bandpass filter. The insertion loss at 435MHz is typically 0.1dB The attenuation at 335MHz is typically 80dB, and continuing down at lower frequencies. Install this, on the tower, at the input of your 70cm pre-amplifier. Water will not harm it or change its characteristics (provide a drain hole). 3. Careful attention to all mechanical joints on your antennae and the tower (make sure all joints are clean and tight) is the solution to this possible cause. A "dirty" joint acts like a diode. It rectifies the Tx 2M signal generating and radiating harmonic energy, which enters the 70cm system with negligible attenuation.
See our advertisement in CQ-VHF for other details.
PS: Don't use a 2M interdigital bandpass filter (our PSf144) on the output of your 2M transmitter. Being a transmission line style filter, it has multiple low loss passbands at odd harmonic frequencies. Its attenuation at 435MHz is negligible!
73s, de John G3BVU/W1.
Yours truly,
C.J.Beanland, MSEE, C.Eng., MIEE, MRAeS. Sales Manager, Spectrum International,Inc. [email protected] P.O. Box 1084, Tel: (978) 263-2145 Concord, Mass. 01742. Fax: (978) 263-7008