I am using a modulator circuit that I found on the web. An Exar function
generator chip provides the modulation. I will probably go with a Twin T
oscillator for the final product. I am using an AD9851 for the RF source.
I had actually thought about some sort of loop antenna. The Wellbrook design
looks pretty interesting. It appears to use a diff amp as the input stage
for their RF amplifier, probably using some sort of CATV transistor for the
final amp. If there is a good market for something like this amplifier, I
could probably develop it. Last year, I did some experiments with a
regenerative RF amplifier but ran into a brick wall trying to find those
linear optoisolators. Maybe I will give that another try.
An interesting about receivers that use those 50 ohm Mini-Circuits
doubly-balanced ring mixers............you can connect an 8 foot diameter
untuned loop directly to the antenna input of a receiver, closing the loop
at the ground connection on the receiver. LW/MW performance is astonishing,
even with no preamplification. As an example, I can tune to 524kHz and hear
the Iowa City beacon up her in northeast Illinois.

Pete

wrote in message
oups.com...
When you say DDS based generators, how do you propose doing the
modulation? I understand how you can get a carrier from DDS, but do
these chips have digital modulation as well? In the dark ages, I did
DDS (very low frequency) using coordics. This was prior to DDSs being
on the market.

Personally, I'd like to see someone compete with Wellbrook on their
loop antennas. The damn George Bush weak dollar policy really sucks
when you buy stuff from the UK. Hell, it sucks just buying gasoline.
Even making the amplifier such as the Wellbrook ALA 100 would be a nice
product.