c1946 Amp, anybody built one?

[mountainman]

put a 10k pot on the input pad from the driver to ground. Currently its reading 10,690 Ohm resistance to ground. Can I harm my driver by running it like this? I haven't turned anything on yet, but I think the fine tuning pot will be the easiest way to tune input power from the driver to the amp.......

IMG_20171028_143248.jpg

[mountainman]

Figured out why the 1946 got hot yesterday. At some point during my case construction, the ground from the ouput coax became disconnected from the board! NO WONDER THE SWR WAS SO BLOODY HIGH!

Gonna fire it up again as soon as someone knowledgable chimes in on whether the pot on the input pad is a good idea.

[radium98]

no npot to rf dear
another thing i get this issue one and it was from the field going around from the rf pa to the brushless fan coils that make it stop and not the power anyway figure and isolate everything as possible by screening
rf and experiments that will make you learn and learn and here in forum the most are expert .

[mountainman]

I fired it up with the pot on and it made zero difference. I was able to re tune/cut 1.5" off the dipole and am getting as best a swr I can at this point; less than 1.2. I'll remove the pot and try to shield the fan.

[sinus trouble]

Hello Again Mr Mountainman
I would avoid using a potentiometer to control drive, they are not designed to dissipate that kind of power! (1/4 watt to 1/2 watt max)
The 1 to 2 watt drive from your CHZ thing should be fine to drive the amplifier direct!
The problem with your fan seems could possibly lay with your DC PSU? Remember that when the amplifier is idle, it will draw very little current hence the fan running! When in operation the amplifier could draw up to 4amps!!
You should check that your PSU is maintaining 13.8v @ full power?

[Albert H]

I fired it up with the pot on and it made zero difference. I was able to re tune/cut 1.5" off the dipole and am getting as best a swr I can at this point; less than 1.2. I'll remove the pot and try to shield the fan.

How are you matching your coax to your dipole? You need some kind of balun if you're not going to waste a significant part of your expensively generated RF!

[Analyser]

Try some ferrite beads on the fan supply wires if the fan supply voltage is ok (after testing as Sinus suggests).

Don't forget to have inlet holes for the fan to suck cold air in to the case, but also outlet holes to let the warm air out (preferably on the other side of the case to the fan).
A lot of people make the mistake of just having a fan in the case with no holes, or just inlet holes. Either way the fan just circulates hot air inside the case.

[mountainman]

I fired it up with the pot on and it made zero difference. I was able to re tune/cut 1.5" off the dipole and am getting as best a swr I can at this point; less than 1.2. I'll remove the pot and try to shield the fan.

How are you matching your coax to your dipole? You need some kind of balun if you're not going to waste a significant part of your expensively generated RF!

I just hooked it up and set the caps to put out 20w, then observed the CB SWR meter whilst cutting a 1/8" off at a time from the Dipole. Because I can't really use the CB meter to actually measure the SWR, I turned the calibration knob until the meter read 2.5:1. I was able to slowly cut away length on the two 16ga legs that make the dipole until the SWR read around 1.6:1. I did another 1/8" cut at this point and the SWR rose a RCH. I figured at this point I had the best match I was gonna get. I added just a little solder to both ends and was able to get it back to the best reading I had gotten before I cut too much. After I got a good length for my freq. I taped up the ends to the PVC and will be painting it a stealthy color later this evening. I like the overall length on the dipole because it will easily sit on my roof rack on the auto without looking out of place. Also very easy to stick a pice

IMG_20171029_160611.jpg

I tried researching an antenna matcher schematic for FM broadcast but found very little on google. On AM (what I'm familiar with), its a pretty simple device involving two variable caps with a coil between them to add capacitance (effectively adding or removing length of antenna). I am unsure why no one has made one to match a transmitter to a specific length antenna by turning some caps in case the operator wanted to change frequency while maintaining optimal SWR.

Hello Again Mr Mountainman
I would avoid using a potentiometer to control drive, they are not designed to dissipate that kind of power! (1/4 watt to 1/2 watt max)
The 1 to 2 watt drive from your CHZ thing should be fine to drive the amplifier direct!
The problem with your fan seems could possibly lay with your DC PSU? Remember that when the amplifier is idle, it will draw very little current hence the fan running! When in operation the amplifier could draw up to 4amps!!
You should check that your PSU is maintaining 13.8v @ full power?

IMG_20171029_161111.jpg

My power supply should be more than adequate, it puts out 30A @ 12vdc. I currently have it set at exactly 12.0VDC, but can easily turn a screw and bring it up to 13.8VDC. This would probably be a good idea as I am planning on running it either mobile in the auto (13.8VDC+) or hooking it up to a deep cycle marine battery and leaving it somewhere high.

Try some ferrite beads on the fan supply wires if the fan supply voltage is ok (after testing as Sinus suggests).



Don't forget to have inlet holes for the fan to suck cold air in to the case, but also outlet holes to let the warm air out (preferably on the other side of the case to the fan).
A lot of people make the mistake of just having a fan in the case with no holes, or just inlet holes. Either way the fan just circulates hot air inside the case.

I have some clip on chokes I can try. It's strange. When the caps are turned so out put is less than 15W, the fan turns. When the output is tuned near the top (25-30W) , the fan stops turning.

I probably don't have quite enough exit holes but I have a few. The fan sucks in, and the holes are on the other end of the box past the heatsink:

IMG_20171029_161304.jpg

IMG_20171029_161257.jpg

[mountainman]

Pretty sure I cooked the 1946. My driver has a high and low setting and I left it on high after doing some late night antenna testing. I fed about 7W into the amp which registered over 100W coming out of it. The transistor got ungodly hot and by the time I realized what had happened, it had been overdriven for about a minute and a half. Now the most I can get out of the amp with 1W of drive is about 8W.

Time to order another (or two) and try again this weekend.....

[Analyser]

Try and get a VHF power meter, these CB things will drive you mad with the crazy readings. I would recommend Daiwa as being reliable and good value. They do some small cross-needle ones with a direct readout of SWR without having to do the CAL thing.

[radium98]

revex also is good if u could find one and up to 200w

[mountainman]

Something like this?

http://www.cheapham.com/daiwa-cn-101-hf ... continued/

I've been noticing that most of the available meters are designed to work from 140-525MHz. A few of the reviews on those meters state they will not work below the rated frequency. I'm willing to put a few bucks into this to get one that will last and work on upcoming larger power output projects, I just want to make sure it's exactly what I need...

[mountainman]

I think I'll end up getting one like this. I can use it for my HF stuff too.

https://www.ebay.com/p/1-8-200-Mfj-822- ... 1739998960

Just a waiting game til' payday now.

I'm probably going to get two of the 2SC1946s. Does anyone know if I can use a splitter/ combiner with one LPF to parallel the two 1946s?

[radium98]

LOL are you kidding it is just my question abose did you ever read what i always write ?

[mountainman]

I hadn't heard an answer yet........

[Albert H]

OK

Feeding dipoles for fun and profit:

A dipole is a balanced aerial. Coaxial cable is an unbalanced feeder. If you just connect the coax straight to the aerial elements, you have a guaranteed mismatch of at least 1.5 : 1 and you'll be wasting up to 30% of your power as heat!

The easiest way to overcome this problem is with a simple "balun" - a network (or transformer) that converts unbalanced to balanced.....

Simplest of all is the Pawsey Stub. All you need is an extra bit of coax! Here's how to do it:

pawsey1to1stub1.GIF

Here is how to figure out an electrical 1/4 wave length of coax:

In Feet:

246 x (Velocity Factor) / Frequency (MHz) = Length in Feet

In Inches: 2952 x (Velocity Factor) / Frequency (MHz) = Length in Inches

Metric formulas for Centimeters:

7500 X (Velocity Factor) / Frequency (Mhz) = Centimeters

With this in place, your dipole cut to the right length and with the aerial at least 3m away from everything, you can get 1 : 1 quite easily!

[mountainman]

OK

Feeding dipoles for fun and profit:

A dipole is a balanced aerial. Coaxial cable is an unbalanced feeder. If you just connect the coax straight to the aerial elements, you have a guaranteed mismatch of at least 1.5 : 1 and you'll be wasting up to 30% of your power as heat!

The easiest way to overcome this problem is with a simple "balun" - a network (or transformer) that converts unbalanced to balanced.....

Simplest of all is the Pawsey Stub. All you need is an extra bit of coax! Here's how to do it:

pawsey1to1stub1.GIF

Here is how to figure out an electrical 1/4 wave length of coax:

In Feet:

246 x (Velocity Factor) / Frequency (MHz) = Length in Feet

In Inches: 2952 x (Velocity Factor) / Frequency (MHz) = Length in Inches

Metric formulas for Centimeters:

7500 X (Velocity Factor) / Frequency (Mhz) = Centimeters

With this in place, your dipole cut to the right length and with the aerial at least 3m away from everything, you can get 1 : 1 quite easily!

So velocity factor should be expressed as ".78" for RG58 right?

[Analyser]

So velocity factor should be expressed as ".78" for RG58 right?

Actually, for the normally available RG58 with a white plastic dielectric Vf is 0.66. If you're using one with foam dielectric it's 0.73 or 0.78.

[Analyser]

I think I'll end up getting one like this. I can use it for my HF stuff too.

https://www.ebay.com/p/1-8-200-Mfj-822- ... 1739998960

Just a waiting game til' payday now.

I'm probably going to get two of the 2SC1946s. Does anyone know if I can use a splitter/ combiner with one LPF to parallel the two 1946s?

MFJ stuff is known as being a bit flimsy but that meter you showed a link to looks like an exact copy of a Diawa meter I used to have, so it might be ok.

You can combine two PAs together before or after the LPF using something called a Wilkinson combiner. Basically this is two pieces of 1/4 wave long 75 ohm coax, one fed from each amp and joined at the output. You're also supposed to use a 100ohm (non-inductive) resistor to take up any imbalance (but some people leave it out) between the output of the PAs. For FM the length of the cable is 51cm or about 20".

Have a look here...

http://www.uksmg.org/content/split.htm