HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: Kage on June 29, 2020, 1714 UTC
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So this radio does not allow MW external antennas. There is supposedly a menu option by pressing a bunch of buttons like a game
controller to get hidden features including temporary MW antenna jack enable while playing but it does not work on my firmware of this radio which is the newest.
The 880 does have an external jack for SW, but it's disconnected from MW circuitry from the schematic I looked at floating around the internet in russian.
I got to thinking besides the obvious loop antenna and inductive coupling, would it work to loop enough turns around a ferrite bar and match it to my longwire outdoor antenna and grounding and place it near the radio on top where its internal ferrite bar antenna is?
Not sure if this would couple correctly or if it needs to be horizontal to the internal bar antenna?
Just thinking of ideas so that I don't have to modify a perfectly fine working radio.
The only other idea I have without going the loop method is to make a simple box to set the radio inside when listening to MW DX and make a loop around the box and match it to the outdoor longwire.
This is really my largest gripe with this radio. The fact that there is no easy way to connect an outdoor antenna for the 100kHz-1710KHz range. Otherwise this would be a killer on LW and MW.
On SW and FM broadcast bands so far I put this radio as my alternative to my ATS909, it's on par for performance so far, but lacks some RF/antenna features, but God damn the audio out of the 880 is impressive and selective bandwidths up to 9kHz on MW makes the thing a great station monitor compared to anything else I own outside of a TRF barndoor wide set. Then there is down to 500Hz for CW even. So for that it's a step ahead of my ATS909. With both these radios I am setup. They have quirks but make good brothers, one gets the job done when the other doesn't.
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You can try what I did, make one of those godzeye AM antennas with a 20pF - 100pF tuning CAP. Here's a much smaller version of what I've built, https://www.youtube.com/watch?v=3sDtmmRHzZc . Here's one from PVC, https://www.youtube.com/watch?v=Wnkf_gQQwwg So, more than one way to make one. Unfortunately, being inside an aluminum sided house, this also brings in more noise that generated within this house. Outside, on the other hand, is a different story. Works super that way. You might have to experiment though which variable capacitor works for the widest part of the AM BC band. Oh... Pinterest has these, https://www.pinterest.com/pin/345158758913671908/ Anyone have more loop-like antennas to show? They do seem to make the AM band more listenable with area-wide T-storm crashes constantly. Oh yeah... Archive.org should have some ideas as well, https://archive.org/search.php?query=am%20loop%20antenna
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There is a mod for the 880 which allows it to use an external antenna for LW. It is located here in the NDB Group:
https://groups.io/g/ndblist/message/105858?p=,,,20,0,0,0::relevance,,pl-880+modification,20,2,0,70214479
I have performed this mod on my 880. It took me about 45 minutes and was pretty easy. This mod allows me to use my standard NDB antenna - an amplified 96" whip - with the 880. While I wouldn't call this setup a DX Machine the sensitivity at LF is greatly enhanced.
I am not sure if you need to be a group member to access this file, but here is the short form procedure. It's possible that this was also posted in another group - poss an ultralight group - but I have not been able to fine it.
1) Open the 880 back.
2) Wind a few turns (I used 5) of fine insulated wire around the end of the ferrite bar at the end near the Antenna Jack.
3) Tack the wire in place with hot melt or other glue.
4) Solder one wire end to the PCB pad near the antenna jack which is connected to the jack ground.
5) Solder the other wire end to the PCB pad which is connected to the jack tip; this is not used normally.
6) Button up the back. Most any antenna can now be plugged into the jack with a mating plug wired to the tip and ground connections.
I did not need to add a LPF between the antenna and RX. Of course, a winding could be temporarily placed on the ferrite bar and patched to the antenna prior to doing any soldering to estimate the appropriate number of turns required for your setup.
Credit for this mod goes to Steve Ratzlaff AA7U and Geir Laastad LA6LU.
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There is a mod for the 880 which allows it to use an external antenna for LW. It is located here in the NDB Group:
https://groups.io/g/ndblist/message/105858?p=,,,20,0,0,0::relevance,,pl-880+modification,20,2,0,70214479
I have performed this mod on my 880. It took me about 45 minutes and was pretty easy. This mod allows me to use my standard NDB antenna - an amplified 96" whip - with the 880. While I wouldn't call this setup a DX Machine the sensitivity at LF is greatly enhanced.
I am not sure if you need to be a group member to access this file, but here is the short form procedure. It's possible that this was also posted in another group - poss an ultralight group - but I have not been able to fine it.
1) Open the 880 back.
2) Wind a few turns (I used 5) of fine insulated wire around the end of the ferrite bar at the end near the Antenna Jack.
3) Tack the wire in place with hot melt or other glue.
4) Solder one wire end to the PCB pad near the antenna jack which is connected to the jack ground.
5) Solder the other wire end to the PCB pad which is connected to the jack tip; this is not used normally.
6) Button up the back. Most any antenna can now be plugged into the jack with a mating plug wired to the tip and ground connections.
I did not need to add a LPF between the antenna and RX. Of course, a winding could be temporarily placed on the ferrite bar and patched to the antenna prior to doing any soldering to estimate the appropriate number of turns required for your setup.
Credit for this mod goes to Steve Ratzlaff AA7U and Geir Laastad LA6LU.
This sounds like a great mod. At this point I don't mind doing the mod either considering the damn radio came defective when I bought it having a rotary encoder not working for the main tuning, I had to open it and reseat the jack going to the tiny PCB to the encoder which was a PITA but it would have cost money to return the defective radio for another one. Works now but ugh.
Makes me wonder if connecting a large outdoor antenna will detune the ferrite bar antenna though?
Might take some thick paper and make a small loop that fits over the ferrite end, remove and tape and wrap the wire around that and then melt some candle wax over it so I can slip it back over the ferrite so I can fine adjust its position and for later easy removal if it doesn't work well.
Anyway, you say the antenna jack tip isn't used but this doesn't make sense to me, the 20' reel antenna the radio comes with is only a TS jack instead of TRS jack, so the ring and the sleeve is shorted and appears to connect to ground, that only leaves the tip of the plug for antenna?
I might still try a decoupled idea too with placing another ferrite bar outside the radio and connecting that to the outdoor antenna to see if it can even work.
edit: just joined and saw the post "The antenna jack on the 880 is really a stereo audio jack, but it's ring connection point is not used on the radio and can be used as LW/MW antenna input.", solved.
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Okay jFarley I just opened it up and tried that mod. What a bitch it was to get 5 turns around the loopstick. Seems my recent model places the original coils more toward the end only leaving me a tiny bit of space to add the 5 turns, but it worked.
MW improvement was profound on my longwire, especially through my 9:1 unun. In fact on highest RF gain setting it overloaded it but this is daytime and a local 1kw station isn't far away. Everything else comes in as good as my ATS909 now on MW using the same external antenna.
I swear there is a new birdie at 720kHz though, but maybe it was always there. If I tune to 719kHz it goes away :o
Might have to make a few special TRS jacks so I can switch between radios easily.
Here is before I added the coil...
https://i.imgur.com/fjaXYsX.jpg
And after..
https://i.imgur.com/Yb3izO8.jpg
I almost wonder if 5 turns is too much being as it's so close to the normal coil, but I'll know for sure tonight when the sun goes down. If anything it will perform wonders on shorter camping wire antennas, but my 130' inverted L is just too much without some padding lol.
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Kage;
Glad it worked for you. Yeah, I had a bit more room on the rod in my 880. On the other hand, the rod butted right up against the case corner and I had to shave some plastic to be able to slip wire onto the rod.
Prior to doing this mod on mine, I could only capture a couple of IL NDBs at night. Afterwards, I was able to easily log most of the common CAN NDBs as well as a cupla dozen USA states.
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Well it's night now. Tried comparing radios and though the modification does work okay on the Tecsun, my outdoor antenna is just too much for MW band unless the RF gain is on lowest, but even switched lowest it still is better than the built in ferrite antenna so that's positive news.
I can see how this modification would kick butt for shorter "on the go" antennas to boost MW, but for a full sized inverted L outside and ground radials it overpowers the radio so bad that on high RF gain it just becomes noise, whereas my ATS909 can handle it without issue.
So the mod works, but is a little limited. It is what it is I guess. Still my second favorite radio I've owned so far. It's really hard to find any portable that beats the ATS909/DX398. No matter what radio I use I find myself going back to it for serious DXing with permanent antenna installs, but for a small backpack radio that has incredible audio and battery life the Tecsun wins hands down. Also I can't get enough of that wide open 9kHz bandwidth on the highest setting, makes MW and SW sound so good, and that 500Hz bandwidth for CW is a winner.
Just wish I could slow down the AGC for SSB, but that's a known issue with the radio that eventually someone will find a fix for I suppose.
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Update:
I eventually did not like the modification using a coil wrapped around the ferrite bar. It overloaded the radio far too easy with any decent outdoor antenna unless it wasn't much longer than the external wire antenna the radio came with.
After studying the schematic of this radio much deeper I discovered that the ferrite bar and associated JFET transistor circuit is nothing more than a wideband MW amplified ferrite bar, and by wrapping a few turns around like the modification calls for what happens is that any signal is amplified through that first stage (Q26) which can be overkill and cause front end overloading with good outdoor antennas.
There are four wires on the ferrite bar, two for the wideband antenna coil, and two for a smaller sense coil to add positive feedback from the JFET output. At that last point wire 3 goes to the next radio stages just like the shortwave antenna section does. Tapping into this point allows for larger outdoor antennas without the overload issues and sensitivity is on par with my ATS909 radio so I went ahead and did this modification of my own instead...
(https://i.imgur.com/KUKA0iJ.jpg)
That is nothing more than a small 0.1uF capacitor wrapped in electrical tape (triangle looking thing) and tapped between the ferrite bar wire 3 and the unused antenna jacks middle ring. I made sure to bend the wires to be near the PCB ground trace so not to cause any capacitive coupling with unwanted parts of the radio.
Tapping the original MW coil at that point does not have any impact on SW reception or FM, or the internal ferrite antenna when ext. ant. disconnected. It is independent of it once the later stages mix the SW and MW/LW antenna inputs together. It is best to only use a TRS "stereo" jack now though to leave the MW middle ring unused if listening to SW.
This allows for much stronger local stations before the radio can overload, and now connected to my 100' outdoor inverted L antenna the performance is identical to my unmodified ATS909/DX398 radio when both radios RF gains are set to high.
Now the radio can be used with external amplified antennas without overload too, and the reception is more evened out across the LW and MW range this way rather than the looped wire modification found online.
PS: I left my original coiled wire modification unsoldered in the radio just in case I ever want to go back to using that instead.
Hope this helps future experimenters :)