Emi,

Those are some wide open questions, so tough to answer.
For example, most high performance antennas are designed to operate well only at
certain frequencies, you apparently are interested in all of 1.7 to 29.99 mhz.
And most antenna systems are meant to drive a 50 ohm antenna port on the radio,
which the PL365 may not have.

The Tecsun PL365 is interesting.
https://www.tecsunradios.com.au/store/product/tecsun-pl365-radio/
They sell an (expensive) outside antenna for the radio:
https://www.tecsunradios.com.au/store/product/tecsun-shortwave-outdoor-antenna/
Tecsun PL365 uses the Silabs Si4735 chip to implement most of the radio:
https://www.silabs.com/documents/public/data-sheets/Si4730-31-34-35-D60.pdf
https://www.silabs.com/documents/public/application-notes/AN332.pdf

Unusual for a consumer SW radio, this may have a roughly 50 ohm input at the 3.5mm antenna jack for MW and LW.
The description for the radio states:
"The Tecsun PL365 is the only radio in the Tecsun range apart from the Tecsun S2000 Desktop Radio to provide an external antenna connection for the AM/MW band. Hence our Q3070 Outdoor MW/SW antenna is perfectly suited to this model."
The Q3070 antenna is suitable for MW and SW, and has a 10:1 impedance transformer
to drive the radio at 50 ohms from an (assumed) 500 ohm long wire antenna.

Not clear from this if the 3.5mm antenna connection is only for MW/LW (below 1.7mhz), or if it also works for SW.
However, page 18 of the manual:
https://www.tecsunradios.com.au/store/wp-content/uploads/2015/12/PL-365-MANUAL.pdf
shows that we should use the 3.5mm jack only for AM/LW (below 1.7mhz)
Their outside antenna for SW is simply a long wire attached to to the PL365 whip antenna

The first page of the Si4735 datasheet shows that FM and SW signals go into one pin of the chip,
and that AM and LW (anything below 1.7mhz) go into two other pins.

From all this, I'd assume the PL365 is not designed to accept a 50 ohm antenna system directly for SW.
And that your best bet is to simply run a wire, long and high, directly into that whip.
And forget the specialized antenna systems when using this radio.
But I could be wrong.

Here's a source for cheap high performance SW radios:
http://fivedash.com/
At $21 that SoftRockLite is a good start, though the front end filter will only allow through a few hundred KHz.
And they are cheap because all the heavy lifting is being done by your computer with soundcard,
and thus not very portable or convenient.

For ham band use with my radios that do have a 50 ohm antenna port, I am quite happy with a resonant EFHW,
pretty much as described in this forum:
https://www.facebook.com/groups/EndFedHalfWaveAntennas/
For a receiver, you could use a much smaller toroid.



Of particular interest to me, that Tecsun webpage states:
"The Tecsun PL365’s RF design has been upgraded and now utilises the SiLabs Si 4735 chipset (the earlier model PL360 utilised the Si 4734 chipset). The new chipset adds Single Side Band (SSB) to the Tecsun PL365 as well as increasing the coverage of the SW band up to 30 MHz. "

A search through the Si4735 datasheet and programming guide shows no mention of SSB or BFO,
so this is almost certainly not supported by the firmware embedded inside the Si4735.
Most unfortunate! This chip is targeting very cheap AM/SW/FM receivers for the consumer market,
but firmware for SSB would have been trivial for them to include.
There's not a whole lot of AM stations worth listening to on the SW bands these days, that stuff has mostly moved to the web.

Since this chip converts directly from RF to baseband I and Q, the most obvious way for them to receive SSB
would be to add some sort of BFO at the same frequency as the incoming RF.
Alternately, it might use the chip to receive at a constant IF frequency of perhaps 9mhz,
with an extra mixer and oscillator up front to convert incoming RF to 9mhz, and add a standard 9mhz bfo.
Some of the SiLabs chips provide a way to see the baseband I and Q data to allow an added custom processor
to decode SSB, but I don't think the Si4735 does this.

Anybody know exactly how Tecsun is implementing SSB?


> Limitations:I can only fabricate f.s. meters, simple swr bridges and attenuators. Antenna analysers with micro
> processors are over me and my budget. Big toroids for baluns are beyond my reach. But i can make air core
> cores as I have an LC meter.

Have fun.

Not clear why you think building a toroid transformer would be beyond your reach.
That LC meter is probably working at one frequency, and that frequency may be very very low,
inter-winding capacitance can mean entirely unexpected results at higher frequencies.
So may take some experimentation, lots of resources on the web to give you a starting point.
There's a lot you could learn by playing with this stuff, and what you might do is totally open ended.
But you probably want a receiver with a 50 ohm antenna port on SW before
seriously exploring antenna systems.

If you really want to know what is going on with your antenna,
then you might do a web search for the nanoVNA.
Use caution, driving the local oscillator from something like the nanoVNA into a radio might blow the radio.
Another reason for caution: This is a rabbit hole that could take years to properly explore.

Jerry, KE7ER