Over the years I've bought a number of HF "walkabout" antennas from mulitband to mono banders and only had much luck with 28Mhz versions from a UK company that no longer make them. The main models I have owned where the MFJ-1899T which was a nightmare to get working and a MFJ- 1820T 14Mhz mono band version which tuned for 15Mhz when checking on a antenna analyzer in the end MFJ sent me another free of charge which was alittle better.
Some time ago I picked up an old 2nd hand Comet CHF-412 from a UK dealer, The only details I could find before buying it was from Universal radio website:
The Comet CHF-412 is a tri-band portable HT antenna for 7, 21 and 144 MHz. It is 2.5 feet (75 cm) tall and is very thin and flexible. It can handle up to 5 watts. VSWR: < 1.5:1. 50 ohm. Black color. This antenna terminates to a BNC, and is shown left. This is a popular antenna for the Yaesu FT-817.
On my FT-817 even with a counterpoise it didn't match on any band with power levels of 2.5watts and only came close on 21Mhz. I also tested it on my Rigexpert and a sweep didn't really see anything close to the bands advertised. It's sat on the shelf for ages and I hate to break or open things up but it didn't work as intended so I decided to find out what the guts were inside that low tube section.
Once I started to break away part of the fiberglass tube I noticed the whip section wasn't even soldered to the mysterious PCB, so that could explain the poor readings! I carefully removed the rest of the tube to find PCB with 5 toroidal's (3 black and 2 red) and two cermic capacitors.
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One side has two red toroids, 2 capacitors and a insulated wire. Having a guess I gather the capacitor (5pf) and insulated wire is to bypass the coils of wire to allow it to work on 144MHz. The second capacitor is 12pf which then connects to the the two red toroids, I need to remove the insulated wire to count the windings.
The opposite side has 3 black toroids, the bottom toroid looks like some form of balun that then connects to the two black toroids. I need to desolder what I think is a balun to work out the windings but the next one has 23 turns and the last one has 22 turns.
I'm trying to work out what side work on what band but at a rough guess the red toroids are for 7Mhz and the blacks ones for 21Mhz but that balun is throwing me off.
My plan is to see if I can modify it so it matches on the bands it was designed for. I'm thinking about removing the capacitors first and replacing them with trimmers to see what this does and then go from there.
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