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Checking SWR with BITx40

Scott Skroch <sskroch@...>
 

My son purchased a BITx40 and we thought it would be a good idea to check the SWR of his home made antenna before transmitting too much.
 
The problem we have run into is that his BITx40 only puts out a fraction of a Watt of power (.016 W) unless we are speaking into the microphone.  We are not able to use our SWR bridge with the low power.
 
1. Is this normal for a BITX40?
 
2. Is there a standard way to increase the power to the antenna for these types of measurements?  (We could just set up a speaker next to the microphone to give consistent white noise input to the mic but this does not seem right.) 
 
 
Voltage measurements were taken with a multimeter on the output leads of a 50 Ohm dummy load.
1.05V without speaking into the microphone (1.05V)^2 / 50 Ohm *.707 = .016 W (RMS power)
18V Loudly speaking into the microphone (18V)^2 / 50 Ohm *.707 = 4.58 W (RMS power)
 
Thanks,
Scott

Gigabyte
 

This is the nature of SSB.  You can't get an accurate read on SSB because it's comes from removing one sideband and carrier from Amplitude Modulation.  So it modulates the amplitude (power) of the signal based on the input.  

You should get a antenna analyzer.  It will give you a picture of the whole range of the antenna and not just one specific frequency.  The Nano VNA is rediculous cheap.  Less than $25 now.  


On Sun, May 17, 2020, 22:46 Scott Skroch <sskroch@...> wrote:
My son purchased a BITx40 and we thought it would be a good idea to check the SWR of his home made antenna before transmitting too much.
 
The problem we have run into is that his BITx40 only puts out a fraction of a Watt of power (.016 W) unless we are speaking into the microphone.  We are not able to use our SWR bridge with the low power.
 
1. Is this normal for a BITX40?
 
2. Is there a standard way to increase the power to the antenna for these types of measurements?  (We could just set up a speaker next to the microphone to give consistent white noise input to the mic but this does not seem right.) 
 
 
Voltage measurements were taken with a multimeter on the output leads of a 50 Ohm dummy load.
1.05V without speaking into the microphone (1.05V)^2 / 50 Ohm *.707 = .016 W (RMS power)
18V Loudly speaking into the microphone (18V)^2 / 50 Ohm *.707 = 4.58 W (RMS power)
 
Thanks,
Scott

Vic WA4THR
 

If you update the software to Allard's sketch he also has a simple mod to add CW capability for the BitX40. Then you can key the rig with a steady carrier to allow checking SWR or use an antenna tuner. You could also try feeding a steady tone into the microphone from your phone or something to get a reading you can check.

=Vic=

Ashhar Farhan
 

you can find an online tone generator and push that into the mic or just hold mic to the PC's speaker.
- f

On Mon, May 18, 2020 at 2:03 PM Vic WA4THR via groups.io <vhklein=ptd.net@groups.io> wrote:
If you update the software to Allard's sketch he also has a simple mod to add CW capability for the BitX40. Then you can key the rig with a steady carrier to allow checking SWR or use an antenna tuner. You could also try feeding a steady tone into the microphone from your phone or something to get a reading you can check.

=Vic=

Robert D. Bowers
 

(Laugh) Generally I'm not the biggest of fans of lots of "bells and whistles", but that is one very good reason why having "CW" on a simple SSB-designed radio is a good idea!

(I cannot work CW due to weird neurological problems... except by computer.  I could transmit by hand - but forget understanding what someone else sent unless that problem decides to take a break for a while!  So IMO CW capability USED TO BE a "bell or whistle"!  {GRIN!})

I'd thought of the audio from a speaker idea... or a tone generator - as long as it's relatively steady in output.  Regardless of the route taken, one point I'd make - still run reduced power!!!  If the match isn't good, that will protect the finals.  I've found that most SWR meters will work fine with only a watt or two, some even less.  Plus I'd add that reduced power also fits within the rules and is respectful of the amateur community near that frequency.  Keep transmissions short and make sure you ID in some way (CW or voice...).

Bob
N4FBZ

On 5/18/20 4:33 AM, Vic WA4THR via groups.io wrote:
If you update the software to Allard's sketch he also has a simple mod to add CW capability for the BitX40. Then you can key the rig with a steady carrier to allow checking SWR or use an antenna tuner. You could also try feeding a steady tone into the microphone from your phone or something to get a reading you can check.

=Vic=

ajparent1/kb1gmx <kb1gmx@...>
 

For SWR checks on SSB radios I've been known to whistle!

However if I need to do that a lot one transistor tone oscillator
switched in does it just fine.

Allison
---------------------------------
No direct email, it goes to bit bucket due address harvesting in groups.IO

Mike Davis
 

There is a smart phone app called tone generator. Just set it for 1500hz and put it up to the mic.