Another project for your review and comments

Discussion in 'The Projects Forum' started by marshallf3, Apr 29, 2011.

  1. marshallf3

    Thread Starter Well-Known Member

    Jul 26, 2010
    2,358
    201
    I've got a problem that's nagged me for ages. There's an outstanding FM radio station I'd love to be able to receive but it's over 75 miles away in Tulsa and due to it's power I'm a good 20+ miles further away from where I can actually start to get a decent signal from it. I went through all the FCC tower data/location/elevation data and found that I could (just barely) get line of ight to it so all I needed was a far better antenna and some sort of high gain amp that had extremely low noise. Anyhow I took it upon myself to design a proper amplifier system & antenna.

    The amp is a two part system with a mast mounted preamp portion and the gain and tuning is controlled by a box inside the house. I'm using high quality RG56 cable for the RF, the power and control voltages going to the mast mount preamp part use regular Cat5 cable.

    Adjustments in the house box part are present for amp gain, input tank circuit tuning and output tank circuit tuning.

    Using a really nice GaAs pHEMT dual gate MOSFET I'm capable of getting +20 dB or gain with a noise level of less than 1 dB in the 88-108 MHz band - amazing for most any RF preamp.

    The antenna is also of my own design and has +11 dB gain of it's own but it's fairly large. It's a three element cubical quad which, when designed to mid frequency (the station is located at 97.5 MHz) presents a 74.7 ohm impedance thus no matching balun is required.

    Here's the neat program you can use to design quad antennas with:
    http://www2.mmae.ucf.edu/~ssd/ham/quadcalc.html

    Anyhow I've got all the parts but haven't written up a formal BOM yet nor made the PC boards as I'm still fighting to find the perfect combination for doing toner transfer. Thought a few might want to look over the design. It may appear to be overkill in a lot of places (and probably is) but I want to make darn sure it doesn't pick up any interference or unnecessarily lose any gain over the 50' run between the two components.

    I'd also double chck the silkscreen component markings just in case I may have goofed but I think it's all 100% correct.

    http://www.innoengr.com/projects/FM_preamp/mast_amp.jpg

    http://www.innoengr.com/projects/FM_preamp/supply_control.jpg

    http://www.innoengr.com/projects/FM_preamp/amp_pcb.dip

    http://www.innoengr.com/projects/FM_preamp/pwr_pcb.dip

    [EDIT:] Well that wasn't very nice of me to pot the PCB layouts in DIP trace format but you'll gt the best output if you intall the free program from http://www.diptrace.com

    Anyhow here they are in .jpg format but I'd be careful about making sure the sizes all match up before you try and etch a boad from .jpg images. The ones that DIP Trace outputs directly are always right on the money.

    http://www.innoengr.com/projects/FM_preamp/amp_pcb.jpg

    http://www.innoengr.com/projects/FM_preamp/pwr_pcb.jpg

    If building this it may pay to add a little shielding to help isolate the output section from the input section of the preamp part. This circuit was built and tested by a friend in another state and he said it worked great, I've got all the parts but just haven't etched the PC boards or put the antenna together yet.
     
    Last edited: Apr 29, 2011
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