Ham transceiver as LW RX

[WA8ZTZ]

Don't worry about it guys, these conversations sometimes tend to wander a bit as we get enthusiastic about all things involving radio.

Anyway, as Scan 125 mentioned, the original intent was to see what actual experience others may have had using the general coverage feature (if available) on their Ham transceiver. My particular interest would be LW performance as LW and even AM BCB seem to be almost somewhat of an afterthought on some of these radios as evidenced by the rather vague specs given in some cases. The VFO may tune down far into the LW region but performance may not necessarily be as good as, say, on the Ham bands.

So, anybody else with experience using your Ham transceiver as a receiver on LW or even AM BCB feel free to share your thoughts.

 

[jwt873]

I built my own low pass filter from plans that I found on the net.. It really cuts out hash from the AM band. 500kHz Low Pass Filter

I put it in a larger case that I had. It's a bit more bulky, but works well http://ve4cy.net/misc-images/lowpass.jpg

It's supposed to cut off at 500 Khz, but when I swept it, the actual cutoff was 800 Khz. It's very sharp there and really knocks down AM stations above that. The 800 Khz cutoff is OK for me because we only have one local station below that and it doesn't cause any problems.

 

[ka3jjz]

A couple of comments are in order here;

a. No sweat with taking this thread a bit off course- that's what I'm here for, hi.

b. Many if not all ham HF transceivers have general coverage receivers built in; and many do go down to the LF ranges.There are some that have a hard wired attenuator (the cutoff is usually somewhere around 1000 khz, but that's not a hard and fast rule) to limit overloading issues. How that affects LW overloading issues is very much in play here

c jwt brings up an excellent point that deserves some stressing. Before you buy or build a filter, do some homework and find out which MW station(s) are the offenders. There might be more than one in some cases. You will need to know what frequencies these are to insure that the filter you buy has a cutoff that nails these.

How do you determine this? Really easy. No test equipment needed. Disconnect EVERYTHING that might act as an antenna, along with the antenna and ground. Now tune the MW band and see which frequencies are still really strong. Easy Peasy. Now plug things back in, 1 at a time. Note which cable(s) cause the offending signals to spike in strength. Now you know which cable(s) may need additional shielding, wrapping around a toroid or similar, or maybe need outright replacing. Note that you can run into a situation where 2 or more cables interact with one another. These can be a bit more challenging; keep notes on what you find. No 2 installations are identical.

Mike.

 

[WA8ZTZ]

How do you determine this? Really easy. No test equipment needed. Disconnect EVERYTHING that might act as an antenna, along with the antenna and ground.

Good advice... recently had a situation here involving poor, noisy LW reception on my RSP1 SDR. Found that the USB cable was a major culprit... apparently poorly shielded. Replaced it with a different, better cable and much of the noise was cleared up.

 

[eorange]

I posted this (exactly) one month ago, regarding my FT-DX1200:

https://forums.radioreference.com/hf-mw-lw-general-discussion/366039-228-khz-harmonic.html

 

[WA8ZTZ]

eorange,

A true image of a signal will be heard at a frequency 2X the IF of the receiver usually below the frequency on which the station transmits.

So, given the second IF of the Yaesu is 455, then:
1140 - 2 (455) = 1140 - 910 = 230


Therefore, you are correct, what you suspect you are hearing near 228 is an image of the station on 1140.

 

[ka3jjz]

Actually images are more common that show up higher, not just lower, than their fundamental. But yes, it can and does happen. I've always suspected that the LW section just doesn't have as good filtering and overload protection than the higher (MW and above) bands. Part of this, I'm sure, has to do with how LW is used in the world. Here in the Americas, it's not used for broadcasting, but in Europe (and to a lesser extent, in northern Africa) it is. This is gradually dying out, though, due to the movement towards FM. Mike

 

[jwt873]

One of the things that my low pass filter (described above in Post #22) cured was interference from stations in the AM broadcast band mixing together directly.

If you mix two frequencies together, you wind up with the original frequencies plus the sum of the two frequencies and the difference of the two frequencies. In my case, there are two nearby AM stations. One at 990 Khz, the other at 1290 Khz. Their towers are close together. (Within 10 miles of each other).

The sum of these is 990 Khz + 1290 Khz = 2280 Khz. That's not a problem. But the difference frequency is 1290 Khz - 990 Khz = 300 Khz.

Because of this I had a strong birdie right at 300 Khz containing low level music and speech. Installing the low pass filter completely eliminated the interference.

 

[ka3jjz]

This is an example of a mixing error, not imaging. Just goes to show you that there can be many ways a MW station can show up on LW...Mike

 

[WA8ZTZ]

Actually images are more common that show up higher, not just lower, than their fundamental. But yes, it can and does happen.

In the case mentioned in my post #26, it looks as though the L.O. is using high side injection ( L.O. freq is higher than the desired signal by the amount of the I.F. freq). Therefore, L.O. freq is 230 + 455 = 685.
Also, 685 + 455 = 1140, so, a strong signal on 1140 ( in this case WRVA) will also mix with the L.O. and be passed on through the I.F. stage. Thus, an image of WRVA 1140 will be heard at 230 which is twice the I.F. BELOW the freq of 1140. This is considered a true image.

 

[WA8ZTZ]

I built my own low pass filter from plans that I found on the net.. It really cuts out hash from the AM band. 500kHz Low Pass Filter

I put it in a larger case that I had. It's a bit more bulky, but works well http://ve4cy.net/misc-images/lowpass.jpg

It's supposed to cut off at 500 Khz, but when I swept it, the actual cutoff was 800 Khz. It's very sharp there and really knocks down AM stations above that. The 800 Khz cutoff is OK for me because we only have one local station below that and it doesn't cause any problems.

Nice looking work !

 

[eorange]

In the case mentioned in my post #26, it looks as though the L.O. is using high side injection ( L.O. freq is higher than the desired signal by the amount of the I.F. freq). Therefore, L.O. freq is 230 + 455 = 685.
Also, 685 + 455 = 1140, so, a strong signal on 1140 ( in this case WRVA) will also mix with the L.O. and be passed on through the I.F. stage. Thus, an image of WRVA 1140 will be heard at 230 which is twice the I.F. BELOW the freq of 1140. This is considered a true image.

Thanks for the math breakdown & that makes sense to me. It's always a great thing when these peculiar observations can be explained.

 

[WA8ZTZ]

Thanks for the math breakdown & that makes sense to me. It's always a great thing when these peculiar observations can be explained.

This one figured out just using basic arithmetic. Some of the mixing and intermod stuff can be figured out mathematically but most of it is beyond me.