With these types of amplifiers it can make a difference....maybe it shouldn't but it does. More so when you start driving one with another. There have been a lot of discussions about the hows and why's over the years. Someome who's never been in that situation with that kind of equipment will always call bs.
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Donkey stomper 2x4
- Thread starter Rob Berger
- Start date
With these types of amplifiers it can make a difference....maybe it shouldn't but it does. More so when you start driving one with another. There have been a lot of discussions about the hows and why's over the years. Someome who's never been in that situation with that kind of equipment will always call bs.
I used to think it was as simple as a bad input match but it can go deeper than that. Why the coax length hides some issues I don't completely understand.
I have found a with some of these amps is the oscillation starts at a certain output level. You'll have a low input swr and everything looks fine until you hit the not so sweet spot. It takes off, power output goes up, input and output swr goes through the roof. This only seems to happen with an antenna connected. Switch to the dummy load and everything is fine at all drive levels and any jumper length.
Sometimes one will break into oscilation with low drive and stop when you increase the drive. Sometimes it's the opposite. Something people miss is what happens with modulation. If the reflect jumps way up with modulation it's a good idea to pull out the scope do some investigation. The positive peaks are a fuzzy blur.
There's not a single root cause. I've found mismatched transistors, caps out of tolerance, bad tuning and bad assembly practices. Most of these amps would play nice with a certian length of coax though.
So I took in all the information I received here and went back through everything. Turns out the jumper from the radio to the amp was to long. I have the radio keying 2 watts. The amp dead keys 200 and talks about 625. If you really get into it and rev the motor I've gotten almost 700 on the bird meter. I want to thank everyone for their input and knowledge. I'll post a couple of pictures of the setup.
Glad it seems to be working at the moment. I would contact the amp builder and see what input wattage he suggests. Most 2x4 amplifiers run with say 3/4 a watt dead key and 6-12 watts PEP. The only way your amp will handle the wattage from a Stryker 655 is if the builder padded the hell out of the input stage of your amp with swamping resistors. All this does is eat up most of the radio's wattage and create lots of heat.
From http://www.k0bg.com/antmount.html
A decent quality (read that as efficient) HF mobile antenna is not a inexpensive commodity, and may cost upwards of $1,200 (although lessor quality ones are about half this amount). Assuming you spent this much for yours, why would you mount it in an inefficient manner? Yet, this is the case 90% of the time. It pays to remember, that Antenna Efficiency is a balancing act between the various system losses. Even one ohm of additional (resistive) loss (anywhere in the system) can make a significant difference in the signal to noise ratio, on both ends of the contact. Thus the antenna should be attached in such a way to maximize what little ground plane a vehicle represents. The key phrase here is, it is the metal mass directly under the antenna, not what's along side that counts!
That flat bar doesn't offer much ground plane.
Eventually the antenna will be mounted centered of the bed behind the cab. I read the whole post in the link you posted. I understand how a mobile antenna works. The reason for the higher mount on this antenna is because the manufacturer specs say the load should be even out above the cab. On my mfj analyzer I have a swr of 1.2 on 27.205 (20) with a X factor of 8. That's pretty good for a mobile antenna. The ohms are a little low @ 47. Since my last post the radio has be returned to a 3/4 watt dead key and about 45 watt swing rms. 20 watts peak on the bird. I do have a voltage drop on the amp I'm currently working on and not sure a large capacitor would help this or not. Currently running at 14.5 volts and dropping to 11.8 at key down full power.