27
votes
Accepted
Which HF bands are best during the day and which are better at night?
HF propagation over long distances is by skywave propagation, the reflection and refraction of radio waves between Earth's surface and the ionosphere. The ionosphere is a consequence of radiation from ...
- 49.9k
19
votes
Accepted
How does selecting the opposite sideband when receiving CW on HF reduce interference?
CW signals are not “transmitted on the upper sideband”, nor the lower one. A CW signal is approximately at a single frequency (with only the additional bandwidth required to allow the key-up and key-...
- 23.1k
15
votes
Which HF bands are best during the day and which are better at night?
In general, the shorter wavelength HF bands are better during the day, and the longer wavelength ones at night. Although that depends a bit on what you want to do, and like all things propagation, it'...
- 5,953
15
votes
Why do operators mention the band when calling CQ on phone?
I have a scanner capable of HF receive, multiple HF receivers, and several HF SDR transceivers capable of receiving on more than one amateur band slice simultaneously. Lots of contest stations run ...
- 12.5k
13
votes
Accepted
What are these continuous sine wave tones I keep hearing?
A pure tone is an unmodulated signal — it carries no data. Almost nobody intentionally transmits a pure tone — it would be wasteful. The exceptions are the time-and-frequency reference signals like ...
- 23.1k
13
votes
Do these look like amateur radio antennas?
It looks like a fibre or telephone cable strung between buildings.
It sets a good precedent for setting up your antenna though - if you can get access to the other rooftop at night, and a catapault or ...
- 8,171
12
votes
Accepted
Why do I only hear broadcasts from 150 kHz to 29.999 MHz?
It doesn't sound like you're doing anything wrong. Most likely, you are simply only hearing strong stations; broadcasters put much more power into their transmissions than amateurs are legally allowed ...
- 23.1k
12
votes
Accepted
What has happened to HF?
HF propagation is not only limited by manmade interference but also by natural phenomena like sunstorms.
Anyway, I'd account that to social and technological reasons:
HF activity suffers very much ...
- 13.4k
11
votes
Accepted
How to do APRS in real wilderness?
So you want to broadcast your location using backpack-portable equipment from deep in a narrow valley hundreds of miles from civilization. I'll assume that you would also like the ability to get a ...
- 8,922
11
votes
Why do operators mention the band when calling CQ on phone?
It may have served a purpose in the past, with less filtering in transmitters and receivers, no digital frequency displays &c, e.g. to avoid mistaking a harmonic on a higher band for the ...
- 612
10
votes
Accepted
Can there be (long range) propagation of HF from satellites?
Satellites can and do use HF for communications.
The first example would of course be Sputnik, which transmitted at 20 and 40 MHz.
Amateurs use HF to communicate with satellites. According to Amsat,
...
- 8,171
10
votes
Accepted
What are the relative bandwidths of the amateur bands?
The usual figure used in this context is, apparently, fractional bandwidth, defined as the bandwidth divided by the center frequency, and therefore having a range of 0 to 2.
$$
\text{fractional ...
- 23.1k
10
votes
antenna in the rain
The antenna itself won't be significantly affected by the water.
However, waterproofing the coax connection is essential. If this is not done, water will creep inside the coax by capillary action and ...
- 49.9k
10
votes
Accepted
Why aren't "ghosts" and intersymbol interference due to unmatched impedance (high SWR) a concern for HF receivers?
Intersymbol Interference (ISI) — or analog equivalents, like ghosting — are relevant only when the difference in time of arrival between the primary and reflected signal are significant compared to ...
- 49.9k
10
votes
Accepted
Using shortwave communication, how difficult is it to transmit messages from the UK to the US?
To send a message of around three bytes over such a long distance, would this require an antenna that uses alot of power?
No. There's a huge variety of tradeoffs here, but as a quick calibration, ...
- 9,021
9
votes
Accepted
Do short waves pass through or reflect off concrete buildings?
The concrete is relatively transparent to radio waves of such large wavelengths (it attenuates, it doesn't reflect). However, steel bars within concrete typically convert that concrete to a solid ...
- 13.4k
9
votes
Accepted
Will electrically joined dipoles of different lengths, at right angles, behave as a multiband antenna?
The results depend on the two bands you choose. Frequency ratios of 2:1 are a good choice because the longer dipole, which is a full wavelength at the higher frequency band, will show high impedance ...
- 7,683
9
votes
Accepted
Is "low loss" cable required for HF?
Your question seems as much about psychology as much as technical concerns. We mainly favor the technical questions, but I'll take a stab at the psychological aspects also. All of you, please feel ...
- 8,922
9
votes
Why do operators mention the band when calling CQ on phone?
Apart from historical reasons it's also a filler. When calling CQ on SSB you probably want to stretch out your transmission a bit because that's what you need to get heard. In theory you could just ...
- 131
8
votes
What's the difference between "maximum usable" and "critical" frequencies?
The maps are related, but as this excellent posting describes:
The 'critical frequency' is the highest frequency that gets reflected
when it is aimed straight up at the ionosphere.
However:
…...
- 6,007
8
votes
Accepted
Why don't monolithic HF transceiver ICs exist?
Why aren't there monolithic HF ICs.
There are, in the shape of actual (broadcast) AM receiver ICs, which can most likely with minimal effort be also used to transmit.
However, are there technical ...
- 13.4k
7
votes
Do I need a Balun for a single band QRP antenna?
You don't need a balun, in that it will work and it won't hurt you. But your antenna will not work as well as it ought to.
When you are transmitting, the effect of a balun used to connect a coax (...
- 23.1k
7
votes
Accepted
What is propagation like when there are no sunspots?
We went through quite an extended sunspot minimum before the current sunspot cycle started, and I well remember what it was like. 10m was open only when there was e-skip, which wasn't very often. ...
- 8,922
7
votes
Why are Wullenweber CDAAs obsolete?
First, there are better DF antennas now and they take up way less real estate. Some of them are .. the Pusher and "L" and "T" arrays. The electronics for these is more sophisticated than the ...
- 139
7
votes
Accepted
Why are Wullenweber CDAAs obsolete?
Two major reasons HF direction finding arrays like that aren't particularly useful any more (you won't find many fixed HF DF stations at all, even more modern ones):
As William said, HF is not used ...
- 1,814
7
votes
Accepted
What are the technical reasons that there are no FM transmissions on the long, medium, or shortwave?
So, the bandwidth argument is the dominant one. You need to incorporate the full Carson Bandwidth in your channel spacing, not only the frequency deviation! As explained in my answer to your previous ...
- 13.4k
7
votes
Accepted
Not-too-distant saltwater ground
With the canal 60 feet away it's not of much help.
The objective with a monopole antenna is not just to have any ground connection, but to have a low-loss ground plane under the base of the antenna. ...
- 49.9k
7
votes
What has happened to HF?
Before looking for reasons why HF activity "appears to be declining", we should look for evidence that HF activity is declining — so let's look at some available sources of data. While contests may ...
- 9,021
7
votes
Will electrically joined dipoles of different lengths, at right angles, behave as a multiband antenna?
What you describe is not far off from a common fan dipole. This is a multiband antenna consisting of several dipoles in parallel, each cut to a different length. Only instead of orienting each dipole ...
- 49.9k
7
votes
Accepted
What is this burst transmission sequence across the entire band?
It's probably an ionosonde — sending out HF radio signals not to communicate but to measure the reflections and thereby determine the current characteristics of the ionosphere.
The signal is probably ...
- 23.1k
Only top scored, non community-wiki answers of a minimum length are eligible