Active antenna for TV...

from a friend just for the hell of it. Am using a

respond as I expected. Am planning to use it for

matter? Please see schematic posted to ABSE under subject

offer (or laughter, if it seems to call for

region with a different front end, but am stumped as to

obviously not an EE, just a tinkerer, and would love

First, to be able to test such things mount antenna outside and wire your signal inside to your workbench via coaxial cable. With rabbit ears you're not working in fixed conditions, it all depends on where your antenna is turned, where your hands are etc. so how can you judge your progress? Besides, it is easier to make an oscillator when antenna is close to amp. Not from your circuit, though, its probably deaf as a fish at VHF.

Next, this amp looks more like MW/SW amp and not too good at that. Probably terrible noise figure and minimal amplification at VHF (if any). You should have amplification at 10MHz, do you have a scope to verify it?

All in all, better to get a good antenna and put it on the roof. There are no magical amplifier designs that will magically pull usefull signals out, if they are below the noise level. Unless your TV receiver has a poor RF frontend you're unlikely to notice much of an improvement, if any.

M

commercial

expected.

MHz

Looks like it'll work OK at audio frequencies so I'd test it down there. (Any!) VHF/UHF response will be entirely down to construction and components. And you've a hope in hell with D1-D2 fitted. Lose 'em, fast!.

Hey M!

Well, you hit the nail on the head with the original purpose of the basic schematic. It was for an active antenna to assist in receiving shortwave broadcasts. :) And it did work for that. I tried using the same schematic and parts for an active TV antenna thinking that since the datasheets for the various JFETs and the NPN transistor indicated that they ought to work for UHF as well as VHF.

10 MHz: no amplification, and I do have a scope (which is how I know there is no amplification and complete signal loss in the second JFET.) I am putting 50mV of 10MHz sine-wave in at the antenna connection point, and it is cut in half by the first JFET, then totally eaten by the second JFET. Could I have overheated the transistors when installing them perhaps? Guess I'll replace them and see. Have checked my connections numerous times, and they appear to be sound and correct. What is it you see that tells you it should be useless at VHF?

Thanks much for the input. Definetly appreciate it...

Dave

Hmmm. I included D1-D2 for static protection, thinking that any actual signal would be too weak to initiate conduction in either diode. What is it I a missing? Seriously asking here, please share.

Thanks,

Dave

would be too weak to initiate conduction in either

If you use "wrong" diodes for the job they have too much capacitance and eat all your signal.

M

schematic. It was for an active antenna to assist in

the same schematic and parts for an active TV antenna

transistor indicated that they ought to work for UHF as well

no amplification and complete signal loss in the

connection point, and it is cut in half by the first JFET,

when installing them perhaps? Guess I'll replace

be sound and correct. What is it you see that tells

Looking at the design it looks more like an audio preamp that might work into the MW/SW range. Try connecting a mic or other audio signal.

I suggest googling for "wideband TV preamplifier schematic" or similar.

M

AAAaahhh. Of course. Didn't even think of that. Any ideas on how to find the right kind of diodes? Could I hook these diodes up to my LCR meter, and get any kind of dependable data for this application?

Much appreciated.

Dave

Thank you. Will do that.

Dave

there.

is

find

and

Only reason I can see for the 3Amp diodes is to help flatten a static discharge if someone touches a Rabbit ear. Me (if forced) 'd just use a couple of BAT85's and I'd change that big, numb, 1000V cap thing to something physically smaller, like a 63V polyester or even a cheap ceramic. Looks like the original circuit was looking to deal with a (single :) lightning bolt but the topology has morphed somewhere down the line. In fact, looking at the circuit and being of a contrary nature, I think I'd lose T1 and T2 and associated bits and pieces and just put the aerial straight on to the 1k pot. The aerial should look like maybe 200ohms, it doesn't need that amount of impedance buffering and consequent buffer losses.

Hey John,

Thank you for the input. Finally looked up the datasheet for the BAT85, and realized it would probably work perfectly where I had the 1N5408s. And

*they* regiseterd 1.7 uF on my LCR meter. Loosing them doubled the singal level delivered to my circuit. And I think I am going to loose the 1000V cap in favor for a 100V version of the same value, also in a ceramic disc. When I used to work on computers (18 years ago) we were constantly warned of the pervasive nature of static, and the impossibly high levels it easily attained. Guess I'm still thinking that way... Like I said, I'm not an EE, just a tinkerer. Obviously.

Your feedback is greatly appreciated.

Dave

[...] Persistence seems the key to electronics :) Had 6.00 minutes idle (waiting for an AVR timeout), so for interest I measured a (similar) 1N5401 using a couple of different LCR meters. One gives a 100mVac~ test signal and saw 80pF (which is about right). The other LCR gives a 0.8V~ test signal and measures similar to what you saw, i.e. the test AC was big enough to be rectified by the diode, thus upsetting the meter. To get a good reading, had to back bias the diode through a 1Mohm resistor connected to a variable DC supply and picked off the test signal via a 100nF cap, before the 2nd LCR meter read correctly, 1V bias=57pF, 30V bias=17pF. Messy setup but shows to be wary of test meters. (Looks like the diode could find good use as a Varicap for a voltage controlled oscillator). Good luck with the amp. john

Wow. Thank you for posting this. Will check out my meter, and see what it uses for the test signal. It's been so long since I've needed to think that way that I don't automatically default to such an analytic mindset anymore. Need to work on that.

Thanks again, and for the good wishes.

Dave