Improving HF Reception
From The RadioReference Wiki
- No matter how good the radio, without an antenna, it won't hear very much. Here are 2 places with lots of links and information on the subject
This term is widely misused in both the ham and listening communities. True antenna tuners are placed at the antenna feedpoint, not at the shack position. They are intended to insure that the antenna delivers as much energy as is possible from the antenna and down the feedline. They usually need a bit of RF to begin the tuning process, so for a listener, they're pretty much useless.
So-called 'antenna tuners', such as those marketed by MFJ, are more correctly called a transmatch. For a ham, a transmatch can be used to overcome difficulties with matching a transceiver to an antenna. This can happen for many reasons, but one that's common is to make an antenna work on a band that isn't matching very well, even though it's supposed to. Doing this without tuning can cause the RF finals to overheat, causing them to cut power, or in extreme cases, be damaged. Another use, depending on the design, is to limit any spurious emissions coming from the transceiver.
For a listener, a transmatch matches the impedance of an antenna to the impedance expected by the receiver. This allows energy to be transferred from the antenna and feedline to the radio, increasing the receive efficiency of the antenna.
Do I need to buy a transmatch that can handle high power?
In a word, no. Units like these are intended for hams running 1 kw of power, or some commercial applications. Many tuners that are designed for QRP (low power) amateur applications will work just fine for listening applications - just don't try to put too much power into them by accident.
What can a transmatch load?
For many years, hams have used transmatches to load things you wouldn't normally consider as an antenna. Things like bed springs, gutters and metal fences have all been used. There's no reason to think that a listener couldn't do the same thing.
What kind of transmatches are there?
There are several different circuits for transmatches; a good Google search on these terms will yield many results. Construction can be thought of as being easy to somewhat complex, depending on the skill of the builder.
- Pi network tuner
- L Tuner
- Tee section Tuner
- Z Match Tuner
In urban areas, it's sometimes necessary to add some front end filtering to eliminate reception (or at least reduce it) of MW signals. These signals can cause multiple false signals (often heard as distorted spurs) to appear throughout parts, or all, of the HF spectrum. This is particularly true of the many wide banded Software Defined Radios (SDRs) on the market.
It's important to evaluate if a filter will solve your problem before you buy it. The better ones have a response curve that will show you how the filter attenuates at different frequencies. A practical example would be something like this; you have a FM broadcaster at 100.3 Mhz that overloads your radio. The unit you are looking at has a response curve that shows attenuation doesn't start until 101 Mhz. Therefore this filter would be largely ineffective in blocking the 100.3 broadcaster. See the filters from PAR electronics and ICE for examples of a response curve diagram.
- DLW Associates BCB Reject Brick Wall High Pass Filter
- Hi Z Antennas Hi Pass Filters
- ICE BCB Filters
- Kiwa Electronics
- PAR BCST-HPF Filter
- Scanner Master Filters page
- Stridsberg Engineering Filters
- Tunable AM Band notch filter
You will see many ads for broad banded and distribution preamps from various distributors, and in places like eBay. Some receivers even have a preamp built in. But just what is a preamp? A preamp is an untuned device that amplifies a wide range of frequencies, usually with no gain control.
This fact can cause considerable headaches, particularly for those living in urban areas with a lot of MW, FM and TV stations in the neighborhood. If your receiver (or SDR) can't handle the additional gain, not only do you risk hearing these stations where they don't belong, the background noise will also increase.
Preamps have their place in certain weak signal work on VHF and UHF (such as Earth-Moon-Earth experiments), but on HF, there is a better alternative...
A preselector can come in two basic flavors...
- Passive- By far, these have the best application, particularly in use with an old time single conversion receiver, and SDRs that often lack robust front ends to reject spurious signals. Passive preselectors have no amplifier, but the tuning is so sharp that they severely attenuate signals from outside the frequency being tuned.
- Active- These units actually have an amplifier with a tuned circuit, and better ones like the Palomar, have a gain control so you can control the amount of gain you apply to the radio
Active preselectors are great for the older desktop radios whose sensitivity tends to drop off around 20 Mhz or so. It does take some practice to use, however. It's a mistake to run an active preselector at 100% gain all the time, because you could still overload the radio, and also increase the noise factor. Listen to the band you're tuning - lower frequencies tend to be noisier. You should consider running it at a very low gain setting, then slowly increase it. In some cases, at some point the automatic gain control in the radio will clamp down on the signal, so no matter how much more you amplify it, you may find it's not getting any stronger. Remember, you are tuning to increase the intelligibility, not increase the noise.
Another way an active preselector could be used is to use it to load a very short dipole - say not more than 1 meter (roughly 3 foot) for each leg. A number of years ago, a company called Datong marketed such an antenna (with a preamp right at the antenna feedpoint) that was popular in Europe and to some lesser extent in the Americas because it's easy to hide the antenna.
Passive preselectors are quite different. You tune a passive preselector until the signal is free from distortion and noise. By definition, you might see a small amount of gain, but nothing like what you would get from an active preselector. These units should be used on SDRs that lack a good front end to resist overloading when a lot of MW, FM or TV signals are in the area. Examples would include many of the SDRPlay units, the RTL-SDRs (with upconverters for HF coverage) and the Funcube. They will work just fine for old desktop communications receivers, too - but without the issues with having too much gain
Please put any additional items here
- Passive Preselectors
- Active Preselectors
- Note: These units were found on Universal's discontinued page. Use them for reference.
- Ameco PCL
- Ameco PCL-P
- Ameco PT
- McKay Dymek DP-40 passive preselector
- McKay Dymek DP-4044 passive preselector
- MFJ 1040B Active preselector
- Palomar P405 receiver preamp
- Palomar P410X transceiver preamp
- RF Systems P-3 passive preselector
- NASWA Journal Review of the RF Systems P-3 (with response curves)
- Return to Decoding the SW Radiogram Broadcasts