Harry,

Many folks used the v3 and v4 boards without mods (and some no doubt still do).?
They don't meet spec with regard to FCC regulations, which require all
spurs and harmonics to be 43 dB down from the main signal.
As I recall, in some cases these undesired transmissions were only 30dB down.
By 30 dB down, we mean that the undesired signal is 10**(30/10) = 1000 times weaker
than the?primary signal of 5 or 10 watts coming out of the uBitx, so perhaps 10 milliwatts.
Regulations in some jurisdictions other than the US are significantly tougher than our 43dB spec.
As far as I know, nobody has actually gotten into trouble because of these issues,
but we should all ensure that our signal is above reproach.

The v3 and v4 board layout for the relays used for switching between the four low pass filters
for transmit allowed some of the harmonics to sneak past the filters.
One fix is to replace those four relays with Axicom relays, which are more suitable for RF.
Search the forum on the groups.io website for "Axicom relays", will find many hits.
An alternative is to add an external low pass filter between the uBitx and your antenna:
? ??/g/BITX20/message/71710
On v5 and v6, Farhan found a way to route the board differently that gets good performance
from the original relays, this fix is working very well.

Once you have the transmit LPF's properly filtering out harmonics,
you are within FCC limits on all bands for CW operation, and all but 15,12, and 10 meters
for SSB operation.? For SSB operation on those high bands, you will have a spur
at 45mhz-DialFreq.? For example if transmitting on 29mhz, there is a spur at 45-29=16mhz.
On the 80,40,30,20,17 meter bands this is not an issue because the transmit low pass filter
will take it out the spur.

Raj found that using a particular brand of shielded inductor fix at L5 and L7 would cure that spur.
Some report it didn't work for them.
Farhan found he could tame the spur on his rig by adding a filter between the 45mhz IF
and the first mixer that prevented the 2'd harmonic of 45mhz from getting into that mixer.
Again, this is a kind of tricky fix.? ? When I search for "spur" in the forum I get 757 hits, mostly
discussions about the above two possible ways to fix this spur.
On v5 and v6, Farhan changed a number of components to reduce the harmonic content
of the signal from the 45mhz IF, this fix is working very well.

My recommendation:
Fix the harmonics, either by replacing the relays with Axicom relays or by using an external low pass filter board.
And don't operate SSB on bands 15,12 and 10 meters to avoid any issue with the spur.

Jerry, KE7ER


On Sat, Dec 21, 2019 at 07:07 PM, Harry Latterman wrote:

I have not have time to build the V3 kit I got in a trade, but plan to soon.? The who thing about the spurs kind of made wonder how many hundreds or even thousands of V3 and V4 radios are out their being used often and the owners are not awared of this issue or maybe some don't even care.? How bad is this problem anyway... Also I was looking at I believe the V5 board and I believe L5 and L7 are not SMT parts but cores.? I guess they seem to work better then the tiny-little parts.? If so why not use them.? This was going to be a future question, but I figure I would ask now...

73 Harry K7ZOV