Why do different shortwave radios pickup different stations?

[KC1LML]

My main thing is collecting and restoring vintage radios. But I also like to use them for DXing shortwave. Recently I've been trying to compare several radios, a 1959 Zenith Transoceanic B600, a 1962 Hallicrafters S-120, and a 2019 Tecsun PL-880. The first 2 radios are tube radios. The Zenith has a RF amplifier. The Hallicrafters doesn't. I did align the Zenith and Hallicrafters after re-capping them.

I'm trying all 3 radios on the same outside antenna, just a simple 60' end-fed random wire with a 1:1 balun. I bring the antenna into the house on a 15' coax. As far as stronger stations, it's hard to tell much difference between these 3 radios. But there is quite a difference on weak stations, especially between the Zenith and the Tecsun. What one picks up the other doesn't and vice-versa.

I would have to say the Zenith is a little better overall than the Tecsun. But the fact is either one can bring a weak station in well enough to understand quite well, while the other doesn't pick up that station at all. The Hallicrafters S-120 was an entry level radio. It doesn't perform quite as well as the Tecsun.

I will say the Tecsun does the best job of the 3 radios as far as eliminating background noise. And of course, it has lots of features the others don't have.

Any thoughts about this?

 

[prima19rider]

interested in the responses

 

[WA8ZTZ]

When doing an over the air comparo between receivers you need a weak but steady signal.
It is best to do such a test during daytime when the signals are least affected by skywave.
No matter how fast you swap receivers/antenna, QSB can be a factor with an unsteady signal.
Also, without knowing the input impedance of the various receivers, it may be that one receiver
could be a better match to the antenna at a particular frequency than another receiver.

 

[pjxii]

Good answer.

 

[Ubbe]

I had an Icom R75 and a RadioShack DX394 connected to a HF splitter and could switch between an active loop and a random wire, and sometimes the R75 received the best signals and sometimes the DX394. It was weak signals in the same band. The R75 have much more control over the signal but at the end they both perform more or less equal but at different times and frequencies.

Probably had to do with how they handled interfering signals or lack of them.

/Ubbe

 

[buddrousa]

LOCATION, receiver sensitivity, antenna

 

[majoco]

I'm thinking that although you have a 1:1 transformer up at the end of the coax, there is still the capacitance of the coax across the radio antenna terminals. Now, depending on the configuration of the RF stages in the receivers, this capacitance may enhance or diminish the "tune" of the antenna circuits. Is there an "Antenna trimmer" or similar control on the S-120 - if so, what effect does that have?

There will always be 'tracking errors' in any radio as the tuning of the antenna coils vs the oscillator coils can never be perfectly in tune across the band unless you want to sit there for hours bending the plates on the tuning capacitors, been there done that - a lesson in frustration - so if the SW bands of the H600 and the S-120 are different then at one particular frequency one radio may have perfect tracking and the other one not so good. The Tecsun tuning is done in software and as long as the software writer did his sums correctly then in theory the alignment should be perfect at any frequency.

Most 'communication' type receivers have a 50ohm antenna socket where yours probably just have a pair of terminals. You might like to try a different transformer up at the end of the coax, say 9:1, to bring the impedance down - or even do away with the coax and bring a single wire down to the radio which was probably what the radios were designed for.

 

[AC9BX]

Antenna input impedance comes to mind. An end fed wire typically has a quite high impedance. Connecting to a low input impedance causes a significant mismatch. For listening only that generally isn't a big problem. Vacuum tube circuits generally have a very high input impedance. The radio's input might some fashion of matching circuit. A solid state radio input impedance could be most anything but a common coaxial input is 50 Ohms. Connecting a wire to a whip on a portable can work quite well but unless they publish it or we measure it we don't know what the impedance is. It should be expected to behave differently.
The antenna's impedance varies with frequency as its length versus wavelength changes.
With a length of feed line from the antenna, coax or parallel wire, impedance mismatch causes losses in the line. This changes as the mismatch changes with frequency. A mismatched line also acts as an impedance transformer and the transformation varies with frequency. Again as frequency changes matching the antenna to the radio changes.
So as you tune around the antenna is more or less effective and that will change from one radio to another if their inputs are not identical.
A minor issue could be the Tecsun being a triple conversion design. It's possible the others have certain frequencies they can't tune well simply due to their design where the Tecsun ideally has fewer such limitations.
Another possibility is non-linearity of the tube models. They may have more gain at different points along their usable signal strength range where the Tecsun is likely very linear.
Are these radios on while next to each other? Yep, that's a thing. Even while antenna is disconnected. The local oscillator(s) in one can affect the other. Tuned circuits when in proximity can also affect the others. An example of that is a tuned loop for the AM broadcast band. One need not make a physical connection. Having the loop near the radio is use is sufficient for a dramatic change in reception.

 

[devicelab]

Probably not so much right now, but from my experience, if the sun is active then it's pretty amazing how propagation can change in a matter of minutes. I did some side-by-side testing of different loops years ago and I had to learn to do them nearly simultaneously. You certainly can't listen to LOOP1 and then 30 minutes later, listen to LOOP2. You'll get crazy results that way.

 

[a417]

What one picks up the other doesn't and vice-versa.

Any thoughts about this?

Each radio has a different design, different implementation, different components and laid out in a different way. It's no surprise each performs differently. If they were all of the same design, form factor and component tolerance then I would be much more concerned that some were malfunctioning, or others were massive over achievers.

 

[GB46]

Are these radios on while next to each other? Yep, that's a thing. Even while antenna is disconnected. The local oscillator(s) in one can affect the other. Tuned circuits when in proximity can also affect the others. An example of that is a tuned loop for the AM broadcast band. One need not make a physical connection. Having the loop near the radio is use is sufficient for a dramatic change in reception.

Interesting! I usually don't connect an external antenna to my portable; the only antenna I have right now is 23 ft. of insulated wire connected to my R75. If I'm tuned to the same station on both radios, disconnecting the antenna from the R75 actually increases the signal strength on the portable, even though I haven't actually connected that wire to the portable. If I do connect it to the portable through the external antenna jack I notice a decrease in signal strength vs. just the whip. However, if I instead wrap a few turns of the wire around the base of the whip, there's a noticeable increase in signal strength vs. the whip.

As for those local oscillators, I found that out a long time ago when I had two AM radios in the same room. They were the older analog radios, each with a 455 kHz IF. Tuning one to 1450 kHz produced an unmodulated carrier on the other radio at 995 kHz. Or maybe it was the reverse; I can't remember exactly, as this was way back in the 1960s.

 

[AC9BX]

If I'm tuned to the same station on both radios, disconnecting the antenna from the R75 actually increases the signal strength on the portable, even though I haven't actually connected that wire to the portable. If I do connect it to the portable through the external antenna jack I notice a decrease in signal strength vs. just the whip. However, if I instead wrap a few turns of the wire around the base of the whip, there's a noticeable increase in signal strength vs. the whip.

As for those local oscillators, I found that out a long time ago when I had two AM radios in the same room. They were the older analog radios, each with a 455 kHz IF. Tuning one to 1450 kHz produced an unmodulated carrier on the other radio at 995 kHz. Or maybe it was the reverse; I can't remember exactly, as this was way back in the 1960s.

Bonkers isn't it? Some of that can be explained. All of it possibly but it's tough to know what's going on.

A tuned circuit in one radio can radiate from itself landing on another radio at some phase relationship to reduce the signal. A regenerative type receiver is good for that, they tend to radiate themselves quite a lot.

Well, 1450-455 = 995. It's not uncommon, especially with older gear, to have a quite strong local oscillator signal and that can do all sorts of things. In governments where a TV viewing license is required, in the UK for example, a method to police it is by using sensitive radio receivers to listen for the IF to determine if a TV is receiving a government channel. (they still sell a black and white license, cheaper than for color)

If using something like a tuned loop for the broadcast band there tends to be a null adjacent to the tuned frequency. If 2 radios are near this but aren't tuned perfectly it's possible to have a better signal on one while poorer on the other.

Of course with scanners especially but for most any radio with single conversion for a strong enough signal one can tune to twice the IF below the desired frequency and hear an image, useful for listening to things the radio won't normally tune. Most radios today can hear very wide swaths but decades ago this would be more prevalent. Maybe you have an old portable multi-band that stops at 470MHz. How can you listen to 470 stations? Tune 21.7 MHz below them, 471.25 becomes 449.55. If it's strong enough you'll hear it fine.

 

[GB46]

A tuned circuit in one radio can radiate from itself landing on another radio at some phase relationship to reduce the signal. A regenerative type receiver is good for that, they tend to radiate themselves quite a lot.

When I was a kid I used to play around with some homebrew regenerative receivers, and on noticing how strong one of them radiated throughout our house I tried connecting a mike into its tuned circuit. The result was a neat little wireless mike, albeit very unstable.