rtl-sdr.com

I just tried and the link works.

I don' know if Minicircuits does mailorder, if not you You have to locate and contact a distributor for prices.

Since you want to buy how about trying SAT-TV splitter e.g. http://www.schwaiger.de/en/4-way-distributor-12-db.html these cover VHF? Comercial splitter combiner are more expensive and price increase with power handling capacity. Another option are directional couplers for providing additional output...

Assuming that you are looking for receiving 137 MHz, did you try searching? Have you checked these pictures https://www.qsl.net/l/lu7did//docs/NOAA ... LU2HAM.pdf ?

Sorted in order of price
absorption meter,
frequency counter,
oscilloscope or
spectrum analyzer available can detect or meassure transmitt frequencies.

How much power do you transmitt and on which bands do you have problems? Have you considered that it is direct RF getting directly into your setup somehwere after you muted the audio that is generating the hiss? I know in todays totally solid state envrionment it is an old fashioned advise, but have...

I assume I understand what you are asking now. You are asking about the effect of conductor diameter onto bandwidth and antenna efficiency. Those are all related to antenna efficiency and bandwith but different effects are involved. It is always a tradeoff what is sensible to do for a given frequenc...

which frequency range, how high AGL and how long (inm) is long for you?
Why .would you want to connect a wire to your PC?

Even If your oscilloscope is fast enough to display strong signals at 137 MHz, oscilloscope are quite insensitive compared to the frontend in spectrum analyzer. The easy solution would be to use a receiver with sufficient bandwidth and sensitivity and connect an IF-output to your oscilloscope.

antenna tuner also can be used as pre-selector tunable bandbass filter

It depends on the design of the matching network if it can be used as pre-selector tunable bandbass filter, e.g a Pi-design is a Low pass filter.

Yes impedance matching circuit/transformer exist. The problem is you have to place such a device directly between the antenna and the transmission line e.g. coax cable to work. Then you have to tune it to the frequency you want to operate on, and again after you change the frequency. This requires s...