[Premium-Rx] General Coverage Antennas for HF + Antenna Preamps

[Ahmet Gundes]

Dear Richard,

I think this issue is another detailed subject by itself.
Desiging broadband IN/OUT matched amplifiers is certainly an
ongoing subject in the industry. 

In most of my experiments with shorter antennas (for receiving
purposes), "perfect" matched amplifiers are not really needed. However when an amplifier is matched IN/OUT to 50 Ohms ( or whatever the system impedance is ) a lower Signal - Noise Ratio results. A remote matching device located at the feed point followed by a transistor amplifier will do the job fine.
In addition the famous formula NF(of entire system)= NF(of the first stage which is the Preamp in this case ) + 1/G x NF(second stage) +.... , where G=Gain of the Preamp.  So in fact the Preamp helps lower the NFigure of the system.

In addition when an antenna is correctly balanced, matched, and
"positioned" towards the incoming signal, there is tremendous
improvement in NF.  I believe the word positioning the antenna
makes mountains of difference in this. If a LOOP for example is
positioned in such a way that it matches the polarization of
the incoming signal then S/N and NF figures improve. Otherwise
the amplifier would be amplifying a signal with lower S/N thus
more noise comes out.

Your suggestion of using an attenuation pad is a good idea for
absorbing the "reflected" signals that could cause IP in the 
transistor. But otherwise I think it would lower the gain.

There are some transistors out there currently being manufactured by Motorola such as BF959 that would provide decent matching with
a 4:1 transformer at the collector with a common emmitter circuit.

Surprisingly Bipolars have better NF than FETs. Also most of the
"Gain Blocks" being made by Mini Circuits, Agilent, Maxim, ... 
have higher NF and their matching is not that great as they are
meant to be used for UHF - Cell Phone freqs. I can also suggest
a commercially available HF Amplifier by AR which sells for $ 43.00
in the US. I works very nicely. However a 2N5179 and some easy to find parts will build an amp just about the same.

usually it is easier to match the output of the amp to 50 Ohms.
However this is fine as long as a matcher is used for the input
between the antenn and the preamp, the matcher will do the job
matching the antenna impedance to the input impedance of the amp.

A High-Pass filter to suppress AM/LW stations at least 60 dB or more is defintely a must. In some cases low VHF TV interferes so
it might be wise to cut the 40MHZ and above as well.

B. regards and a good weekend to all.

Ahmet Gundes



Subject: Re:RE: [Premium-Rx] General Coverage Antennas for HF

> Thank you for that information - just what I was looking for.
> 
> I guess for it to work correctly into any receiver, the amp must see a constant
> impedance of 50R - so  would fit a pad (of around 5-6dB or so) between the amp
> and receiver?
> 
> Richard
> ***************************************************************
> Richard Reich    
> Principal Engineer Hardware
> SAAT Technology Ltd
> Web: http://www.saatt.co.uk
> 
> DDI: +44(0)1420 545388
> Fax: +44(0)1420 87259
> 
>.
> 
-- 
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