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fibber
14-11-2005, 17:38
Are there recommendations for using a preamp at a fixed location? If so what is the cheapest way to go that will still be effective, or is there such a beast? I'm not afraid of a kit if it is fairly easy to assemble.

dk8cb
14-11-2005, 21:01
Why would you want to use a preamplifier? Today's receivers are usually sensitive enough unless a very short antenna such as a short telescopic antenna or a small loop is used.

Also, unless you have a very good receiver, even connecting a good wideband antenna to a receiver often leads to unwanted products generated in the receiver as a result of insufficient large signal performance and too much signal from the antenna. In many cases, it is actually much better to use attenuation to decrease the amount of signal the receiver will have to digest than to further aggravate the problem by even more signal.
Better put up a good antenna and forget about preamplifiers.

Roland

df8uo
15-11-2005, 20:16
Hi Christopher,
i fully agree with Roland. I also use here a FT817 for DRM as you do. The sensitivity is absolutely sufficient. No need for a preamp, when i use my outdoor dipole antennas.
However if you are looking for a simple small active antenna, have a look to
http://www.elexs.de/drm6.htm
I assembled this fairly simple antenna circuit in short time. I spent less than a few bucks ;-) and it works quite good.
73
Daniel

P.S. I forgot that the article is in German, however the diagramm is international ;-) (Empfänger=Receiver)

dk8cb
15-11-2005, 20:44
Originally posted by df8uo
... however the diagram is international ;-)

Not quite. In the US, resistors are drawn differently than they are in (good) "old Europe". :D

Actually, they used to be drawn in the US manner in Europe as well but that was in the twenties of the last century. :p
Nowadays, the US resistor symbol is reserved for wire resistors in Europe but even for these, it is used only rarely.

Roland

fibber
16-11-2005, 02:00
I certainly appreciate all of the comments and the neat antenna plans. I have some anecdotal experiences to share however. I connected my R-7 (multi-band ham vertical) to a broadband 10dB preamp covering 5-900MHz. I also placed a MW filter inline. This then went into the front connector of my FT-817.

Remember that most of you in Europe are in a MUCH higher signal area than those of us here in the USA (particularly the North Western part). To my knowledge, no broadcasters are transmitting DRM to my area (perhaps Kuwait claims to be). We are technically out of the CBC Sackville beam.

Right away I saw a .5 to 2 point increase in my SNR. This resulted in decoded audio that was absent without the unit plugged in.

I also tested my 80M dipole with and without the amp. Although the amp helped, the dipole resulted in signals 5 to 10 SNR points down.

So current status is with the amp and the vertical. Today's DRM from CBC Sackville was one of my best days for the last 3 months.

I admit that there is little science to my goofing, but since a placebo can have positive effects in medicine, perhaps the preamp is "fooling" me, my equipment and software ;)

dk8cb
16-11-2005, 10:10
Originally posted by fibber
I also placed a MW filter inline.

Have you also tried with the MW filter alone? Any improvement in SNR then?

Roland

fibber
17-11-2005, 00:13
I tried today using the weaker signal of MOI Kuwait. There seems to be SNR drop as soon as I take off the preamp. Today it looked more like 2-3 dB. The MW filter didn't seem to have any effect in or out of line, with or without the preamp.

For the time being I'm going to run with the preamp. Any other thoughts on testing?

FritzWue
17-11-2005, 07:15
Hi Christopher,

just as an info here is my amplifier I use in the car with a 5/8L for 2m as whip.
The design is for short whips, not for near 50 ohm input.
As there is dc coupling between the two transistors the input voltage divider of the BF245 controls the current through the BF199. This has to be optimized individually to give about 8 to 9volts at the BF199 collector with 12 to 14volts supply.

dk8cb
17-11-2005, 07:21
Originally posted by fibber
Any other thoughts on testing?

Yes, perhaps it's only a matter of signal level. Can you increase the level at the input of your 12 kHz downconverter, perhaps by turning it up a bit more?

Roland

fibber
17-11-2005, 14:53
I have the 455k>12k board as high as it will go. I think my levels into the soundcard are fine. I get some "red" on DReaM and have to back it down a bit sometimes. Seems close to -20 most of the time.

I have had an SNR without the amp of 31.2db. I often get 28-29 or so. Signals are just so darn variable. I'm still convinced that we are out of the beam of most of the broadcasters and truly must rely on propagation.

As I see it, Europe's relatively smaller area is redundantly covered by multipl broadcasters. We have to really work to copy the 2 or 3 broadcasters that even make it over this way. The nearest one is 4500km away!

I'm still holding out hope that RNZI signals will make it across the Pacific.
I'm the one that thought an email or letter writing campaign might help. DRM needs support in the USA and there seem to be very few listeners currently.

I am always a bit pessimistic though :(