"Low Noise" Vertical Antennas for Receiving

Useful information regarding antennas for SDR products.
sdrom33
Posts: 123
Joined: Sun Dec 09, 2018 4:38 pm

Re: "Low Noise" Vertical Antennas for Receiving

Post by sdrom33 » Tue Aug 06, 2019 5:34 am

Ernestyn, unfortunately this antenna does not deliver what it claims for many reasons. Since you did not get any replies, here I am with my modest opinion.
- It is not a capacitive probe because of its form factor (very small surface) and because, as you rightly note, is referenced to ground.
- the transformer primary is claimed to be high impedance, but it cannot be, because of it's parasitic capacitance to ground, which cannot be eliminated. With the turns given, we can expect anything from 140 pF up.
- The zip cord impedance is not certain, but a matter of speculation, as it depends on
materials used. The slightest imbalance will make it become part of the antenna itself, so once built, nobody knows how the incoming signal is collected.
- The UNDEFINED ground reference, plus the parasitic capacitance of T1 to the ground of the common base amplifier will play havoc with local noise. This amplifier stands on a "floating" ground, but what about its power supply? In such an arrangement the type, layout and capacitance to "stake" ground will produce as many different antennas as there are different supplies that might be used.
- The common base push-pull amplifier by definition is VERY LOW INPUT IMPEDANCE. Why? This puts undue stress on the turns radio of T1 for no logical reason. Furthermore the tapped inductors on the collectors make no sense and render the circuit very prone to parasitic oscillations, which will introduce extraneous noise and interference. A simple center tap on the primary of T2 should make more sense.

To conclude, not only there is nothing on this "design" that justifies the " low noise" claim, but even elementary circuit analysis shows there cannot be any wide bandwith as well, because the parasitic capacitance of T will also create resonances with the forgotten whip inductance which is always there.....
Last edited by sdrom33 on Tue Aug 06, 2019 4:05 pm, edited 2 times in total.

sdrom33
Posts: 123
Joined: Sun Dec 09, 2018 4:38 pm

Re: "Low Noise" Vertical Antennas for Receiving

Post by sdrom33 » Tue Aug 06, 2019 5:48 am

But the worst problem of this design is the use of a transistor amplifier. This warrants a separate post. Even though not in the way intended by its designer, still the antenna will pick up some signal. In presence of strong local signals the amplifier will overload, producing any kind of intermodulation, crossmodulation and spurious products, plus it's own parasitic oscillations which will appear in one unit and not in another, because of different physical layouts. These characteristics make the design completely unpredictable and it's actual operation matter for speculation.
Last edited by sdrom33 on Tue Aug 06, 2019 4:06 pm, edited 1 time in total.

ernestyn
Posts: 14
Joined: Mon Jul 22, 2019 1:40 pm

Re: "Low Noise" Vertical Antennas for Receiving

Post by ernestyn » Sat Aug 10, 2019 5:15 pm

Dear sdrom33, thank for clarification of tbe stylus antenna. I was dubting it and asked for explanation and agree cannot work. For Wideband you need large thick shape but here very thin. Also difficult other antenna capacitor type so small. I am working with capacitor antenna big size and special shape. I will describe later
Last edited by ernestyn on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
Reason: No reason

Post Reply