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Is information available on the changes needed to put the BITX40 on 80 Meters?
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Ryan has posted how to do it here by bypassing the filter? This works but to get a better signal you will need to change/modify the bandpass filter. I may try some from QRP Labs.
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Yep, a filter bypass is dirty, but it works! I'll be using a QRP Labs 80m filter. I think I have devised a way to make it very easy to insert into the circuit with nothing but a switch to choose the filter. I need to do some experimenting though. I'll post information when I have it. The stock LPF should be okay though since the next harmonic when running the vfo down to 3.8-3.99mhz should be over 8mhz, which is above what he LPF lets through.?
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Ryan Flowers KC7RYS?
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What are the values of the capacitor to be used in the filter for 80 meter . I dont want to by the filter because qrn and grm is very high in our location . Thanks in advance . Vu2rjw Soundar
On Mar 27, 2017 7:13 AM, "Ryan Flowers" < geocrasher@...> wrote: Yep, a filter bypass is dirty, but it works! I'll be using a QRP Labs 80m filter. I think I have devised a way to make it very easy to insert into the circuit with nothing but a switch to choose the filter. I need to do some experimenting though. I'll post information when I have it. The stock LPF should be okay though since the next harmonic when running the vfo down to 3.8-3.99mhz should be over 8mhz, which is above what he LPF lets through.?
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Ryan Flowers KC7RYS?
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You might adopt the inductor and capacitor valuesfor 80m? from the schematics of any standard rig.Afterall it is for personal use.
On Monday, 27 March 2017 5:44 PM, Soundararajan Ra. <vu2rjw@...> wrote:
What are the values of the capacitor to be used in the filter for 80 meter . I dont want to by the filter because qrn and grm is very high in our location . Thanks in advance . Vu2rjw Soundar
On Mar 27, 2017 7:13 AM, "Ryan Flowers" < geocrasher@...> wrote: Yep, a filter bypass is dirty, but it works! I'll be using a QRP Labs 80m filter. I think I have devised a way to make it very easy to insert into the circuit with nothing but a switch to choose the filter. I need to do some experimenting though. I'll post information when I have it. The stock LPF should be okay though since the next harmonic when running the vfo down to 3.8-3.99mhz should be over 8mhz, which is above what he LPF lets through.?
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Ryan Flowers KC7RYS?
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There is some information floating around to bypass the bandpass filte but I went digging this weekend and found an ARRL write up for filters. ?I have some pictures of what I did. ?I tombstoned C1 and inserted a switch between the coil side and C1 and I cut trace at the end of the filter for the other end of the switch. ?I made a small pcb with the filter and it works without bypassing the bandpass for a dirty hack. ?Hope this helps. 
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Steve, I've got an 80M filter on the way, and I'm going to do something similar. At the top of the picture where you've cut the line and put in two wires, I'm going to do that, and run those wires to the 80m filter, and of course ground the filter at its ground point. Then I'll use a switch to jumper over the filter I don't want, just like I'm jumpering over the 40m filter now.
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Ryan Flowers KC7RYS?
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The reason I did it the way i did was so that i could still switch the 40M filter in and out 1 less filter to make
On Tue, Mar 28, 2017 at 8:27 PM, Ryan Flowers <geocrasher@...> wrote: Steve, I've got an 80M filter on the way, and I'm going to do something similar. At the top of the picture where you've cut the line and put in two wires, I'm going to do that, and run those wires to the 80m filter, and of course ground the filter at its ground point. Then I'll use a switch to jumper over the filter I don't want, just like I'm jumpering over the 40m filter now.
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Ryan Flowers KC7RYS?
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NM i understand now i was thinking the other end, i cut in the wrong spot the first time and repaired the trace
On Tue, Mar 28, 2017 at 8:35 PM, Steve Greer <km4ous@...> wrote: The reason I did it the way i did was so that i could still switch the 40M filter in and out 1 less filter to make
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Yep I'm leaving the 40m filter in place too. I'll be sure to write it up on my blog when it's all done. A pretty easy mod overall, although I've never built a filter. Should be fun without any real test equipment lol. Do you have any more pictures of your mod? --
Ryan Flowers KC7RYS?
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Hi all,
I have done some experimenting with RFSIM, modelling the existing BPF, and analysing it. I have come up with an easy way to have some filtering, without just shorting out the BPF.
If you put a 470pf from the antenna input to the junction of L3 and C6, you will get a reasonable BPF at 80m.
Change it to a 6.3uh inductor , and you will get 30m. A 2.2uh will give you 20m.
These are not optimal as in the original nice 40m BPF, but an easy way to get at least some filtering from BCB and higher up. See attached pics.
P.S I have tried it, and it works great.!
Cheers
John de VK4KWT
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Thanks for info.
Wondering if you had a similar solution for the lpf?
Joe
VE1BWV
On Mon, Nov 6, 2017, 5:28 PM radiomanjwt < jwtitmus@...> wrote: Hi all,
I have done some experimenting with RFSIM, modelling the existing BPF, and analysing it. I have come up with an easy way to have some filtering, without just shorting out the BPF.
If you put a 470pf from the antenna input to the junction of L3 and C6, you will get a reasonable BPF at 80m.
Change it to a 6.3uh inductor , and you will get 30m. A 2.2uh will give you 20m.
These are not optimal as in the original nice 40m BPF, but an easy way to get at least some filtering from BCB and higher up. See attached pics.
P.S I have tried it, and it works great.!
Cheers
John de VK4KWT
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toggle quoted message
Show quoted text
Hi all,
I have done some experimenting with RFSIM, modelling the existing BPF, and analysing it. I have come up with an easy way to have some filtering, without just shorting out the BPF.
If you put a 470pf from the antenna input to the junction of L3 and C6, you will get a reasonable BPF at 80m.
Change it to a 6.3uh inductor , and you will get 30m. A 2.2uh will give you 20m.
These are not optimal as in the original nice 40m BPF, but an easy way to get at least some filtering from BCB and higher up. See attached pics.
P.S I have tried it, and it works great.!
Cheers
John de VK4KWT
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Hi Joe, I have not tried re modelling the lpf yet, but will do when i get more time. Btw the software is called rfsim, and is free if you wanto have a go. Cheers John de VK4JWT?
Sent from my SAMSUNG Galaxy S7 edge on the Telstra Mobile Network
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-------- Original message -------- From: Joe <joeman2116@...> Date: 7/11/17 7:56 am (GMT+10:00) Subject: Re: [BITX20] 80 Meters on BITX40?
Thanks for info.
Wondering if you had a similar solution for the lpf?
Joe
VE1BWV
On Mon, Nov 6, 2017, 5:28 PM radiomanjwt < jwtitmus@...> wrote: Hi all,
I have done some experimenting with RFSIM, modelling the existing BPF, and analysing it. I have come up with an easy way to have some filtering, without just shorting out the BPF.
If you put a 470pf from the antenna input to the junction of L3 and C6, you will get a reasonable BPF at 80m.
Change it to a 6.3uh inductor , and you will get 30m. A 2.2uh will give you 20m.
These are not optimal as in the original nice 40m BPF, but an easy way to get at least some filtering from BCB and higher up. See attached pics.
P.S I have tried it, and it works great.!
Cheers
John de VK4KWT
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Further to the LPF modification for 80m, I have done more experimenting and found that if you switch in a 470pf cap across L6 in the LPF, you will get about 37db attenuation on 40m, with a nice rolloff from 80m.?
The attenuation on 80m is negligible. See the attached graph.
Cheers
John de VK4JWT
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I have done some more tests with a spectrum analyser, and measured the following levels with a 470pf cap across L7 in the LPF, and a 470pf cap from the left side of L1 to the right side of L3 in the BPF.
See the attached pic, I have used 2 relays, one for the LPF, and one for the BPF.
3.660 +35dbm (5w)
7.320 +10dbm (10mw)
10.980 +10dbm (10mw)
14.640 +2dbm (1.5mw)
18.300 -2dbm
21.960 -7dbm
and so on.
Without the cap mod on the LPF I was getting +27dbm out on 40 metres when transmitting on 80!
I agree it is not optimal, but it is a lot better than trying to use the unmodified 40m LPF.
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Would you have a diagram ?
Thanks
Joe VE1BWV
On Thu, Nov 9, 2017 at 11:16 PM, radiomanjwt <jwtitmus@...> wrote: I have done some more tests with a spectrum analyser, and measured the following levels with a 470pf cap across L7 in the LPF, and a 470pf cap from the left side of L1 to the right side of L3 in the BPF.
See the attached pic, I have used 2 relays, one for the LPF, and one for the BPF.
3.660 +35dbm (5w)
7.320 +10dbm (10mw)
10.980 +10dbm (10mw)
14.640 +2dbm (1.5mw)
18.300 -2dbm
21.960 -7dbm
and so on.
Without the cap mod on the LPF I was getting +27dbm out on 40 metres when transmitting on 80!
I agree it is not optimal, but it is a lot better than trying to use the unmodified 40m LPF.
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Soundararajan Ra.
radiomanjwt
