Basic Questions About Antenna Set-Up for Shortwave

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BigJim
Posts: 2
Joined: Thu Aug 31, 2017 1:32 pm

Basic Questions About Antenna Set-Up for Shortwave

Post by BigJim » Thu Aug 31, 2017 1:50 pm

Hi all,

I'd value your advice on a few queries I have.

I have been playing with RTL-SDR for some time in the VHF and UHF range, but now I am looking for my first foray into shortwave DXing (receive only) with the Spyverter + my RTL-SDR. My goal is general scanning of shortwave, mostly between 5000khz and 15000 khz.

I am fortunate that I have a large plot of land with a few strategically placed trees, so a long wire antenna would be no problem for me.
I live out in a fairly rural area. Other than some small towns several miles away, the nearest city is about 20 miles, so there isn't that much in the way of interference. My house is surrounded by fields in my immediate area, so no neighbours to get RF interference from.

Would I be right in thinking that a half-wave dipole with a balun would generally get a better reception than an end fed wire with an unun?

If I am receiving only, would I need to have an autotuner for a wire antenna?

How do the active antennas like the boni-whip and Wellbrook compare to a half-wave dipole wire antenna? There seems to be a lot of positive reviews about these active antennas, but I'm unsure if that is mostly marketing and vogue, or whether they really are much better for general scanning than a wire antenna?

Thanks!

snn47
Posts: 153
Joined: Tue Dec 27, 2016 11:00 pm

Re: Basic Questions About Antenna Set-Up for Shortwave

Post by snn47 » Fri Sep 01, 2017 12:35 am

It depends on what type of stations you want to receive, the antenna polarization and radiated power used, distance and direction to your location, what type of antenna is optimal for your needs.

If signals are strong even a few meter of cooper wire from an old transformer should work well enough without spending any money. If you want best performance antennas are extremely complex, and are mostly promises fulfilled only if defined conditions exist and are broadbanded.

Without going to much into details, there is no passive antenna that will cover 5 to 15 MHz and provide the same performance within the range. Antennas are resonant at multiples of ¼ of the wavelength, therefore 15 MHz is a third of 5 MHz in wavelength, and an antenna would be resonant on both, but the antenna pattern will differ somewhat at the higher frequency. This does not mean you will not receive not in between, only you cannot assume that the antennas gain, antenna pattern and directivity is as described when they are resonant. But if you want to receive weak signals the antenna has to be directive, have the same polarization, resonant and directed toward the signal source, which is why HAM’s use rotors to rotate their antennas or use a number of fixed antennas and the select the one best toward the signal source.

Antenna turners provide matching of the antennas impedance to the receivers impedance for optimal sigal power transfer. Since tuned/tuneable circuits are involved they can provide some limited filtering.
If you want to receive weak signals in the presence of strong signals you most likely will need filters to suppress stronger signals on other frequencies so your receiver won’t overload.

I had to look up what type the active antennas you mention and found a description here e.g. here
https://translate.google.com/translate? ... edit-text= Both are not an antenna since they are not resonant at a wavelength. Therefore they do not have directivity or a defined polarization, because they are extremely short compared to the wavelength, and use the amplifier in my understanding to match the short capacity the whip provides to the receivers input impedance. Short whip (to the wavelength) active AM radio car antennas used to do for a long time, and could provide an option if their circuit also works on shortwave. The need for selectivity may be lower since signal strength provided is lower.

Then there are differences in how signals are propagated over the day time, changing with sun spot activity, if you receive them by ground wave, or by other propagation directly on the shortest route while sometimes the indirect long way around the earth works too.

Therefore as proposed I would start with stringing a number of cooper wires of different length vertically (with some ground wires called ground plane)or horizontally in different direction directly and connect them, with a simple plug at the receiver port, and see if you can receive the signals you want to rceive. If you cannot than it’s time to read up and experiment further. For all you can find guidance in HAM radio literature and blogs.

Enjoy beeing a SWL

PS.: I would built myself a tuneable magnetic loops on a rotor, bend out of the rigid cooper tubing on a rotor to cover the frequency range and provide selective to enable weak signal reception. You have to retune the antenna with frequency change.

Unlike with any other antenna you can, due to directivity and selectivity, receive uninterfered even in presence of the strongest signals with, like you do with an FM Radio. I used sucessfully a tuneable magnetic antenna (1.2 m diameter) down to 7 MHz,just fixed in a small vice on the top of my my car when parked for HAM radio contacts. When it rained resonance changed and required just some retuning

BigJim
Posts: 2
Joined: Thu Aug 31, 2017 1:32 pm

Re: Basic Questions About Antenna Set-Up for Shortwave

Post by BigJim » Fri Sep 01, 2017 4:34 pm

Hi snn47,

Thanks for your detailed reply.

A wire sounds like a good starting point then. Were you thinking about something like this? http://www.amateurradio.bz/images/quart ... rtical.gif

snn47
Posts: 153
Joined: Tue Dec 27, 2016 11:00 pm

Re: Basic Questions About Antenna Set-Up for Shortwave

Post by snn47 » Mon Sep 11, 2017 4:34 am

For a vertical antenna setup for reception only yes.

You have to keep in mind that like most antenna this ground plane is not broadband, but at SW a few hundred kHz at best, if used as transmitt antenna and this is what this design is for.

Since you want to receive, a wide range use, cut the wire for the 1/4 wave length of lowest wavelength. You still will receive the range and still receive even lower frequencies, but it is not optimal any more.

If you want to optimize think about running a few more ground wires, hang serveral vertical wires, each for different wavelength and make them switchable /use a banana plug at the end of each wire to allow you to optimize the antenna. If you want to try lower frequencies you can string a Long Piece to another tree horizontally and for the rest go vertically down to the feed Point of this ground plane.

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