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6 Meter Harmonics Problem


Jeff Wilson
 

I am a newby to 6M since getting my used K3 (sn 1900) that was updated by Elecraft in August 2018.
Just installed a hombrew 6m 3 element yagi (LB Cebik design) and SWR is great for CW and FT8 50 to 51 MHz.  No amp.
So my problem is RFI at my house about 200ft away.  FM radio going silent and Yagi fed OTA digital TVs freezing (RG6 feed) when I transmit.
Before I build a Low Pass filter for the K3 (like K8CU design QST, Nov 2002 p 34-36), does anyone have similar RFI experience on 6M.
Or should I focus on High Pass filters on the home antennas (what a pain).
My wife loves CBC Classical on 100.5 MHz...almost perfect 2nd harmonic of FT8 on 50.313!
Any suggestions from this amazing group would be appreciated to restore my marital bliss!
73 Jeff VE3CV


Michael Kopec
 

Hello Jeff. My TV antenna is on the same mast as my 6m antenna. (see my QRZ page) I went the hi-pass filter route( Drake w a cut-off of 54 MHZ) at the antenna feed entering the house w a good ground attached to it. Also up graded to quad shield RG6 cable feeding all 5 TV's. I also installed ferrets on each TV power cord. XYL is now happy and I can run 500w on 6M with my K3s/KPA500. Cheers and Beers de Mike K8NS

On Wednesday, June 10, 2020, 08:14:30 AM EDT, Jeff Wilson via groups.io <jjw5257@...> wrote:


I am a newby to 6M since getting my used K3 (sn 1900) that was updated by Elecraft in August 2018.
Just installed a hombrew 6m 3 element yagi (LB Cebik design) and SWR is great for CW and FT8 50 to 51 MHz.  No amp.
So my problem is RFI at my house about 200ft away.  FM radio going silent and Yagi fed OTA digital TVs freezing (RG6 feed) when I transmit.
Before I build a Low Pass filter for the K3 (like K8CU design QST, Nov 2002 p 34-36), does anyone have similar RFI experience on 6M.
Or should I focus on High Pass filters on the home antennas (what a pain).
My wife loves CBC Classical on 100.5 MHz...almost perfect 2nd harmonic of FT8 on 50.313!
Any suggestions from this amazing group would be appreciated to restore my marital bliss!
73 Jeff VE3CV


mikerodgerske5gbc
 

You better hurry because 6 will magically disappear in about 30 days 

7!
Absolutely Awesome 

On Jun 10, 2020, at 7:43 AM, Michael Kopec via groups.io <michaelkopec1@...> wrote:

Hello Jeff. My TV antenna is on the same mast as my 6m antenna. (see my QRZ page) I went the hi-pass filter route( Drake w a cut-off of 54 MHZ) at the antenna feed entering the house w a good ground attached to it. Also up graded to quad shield RG6 cable feeding all 5 TV's. I also installed ferrets on each TV power cord. XYL is now happy and I can run 500w on 6M with my K3s/KPA500. Cheers and Beers de Mike K8NS
On Wednesday, June 10, 2020, 08:14:30 AM EDT, Jeff Wilson via groups.io <jjw5257@...> wrote:


I am a newby to 6M since getting my used K3 (sn 1900) that was updated by Elecraft in August 2018.
Just installed a hombrew 6m 3 element yagi (LB Cebik design) and SWR is great for CW and FT8 50 to 51 MHz.  No amp.
So my problem is RFI at my house about 200ft away.  FM radio going silent and Yagi fed OTA digital TVs freezing (RG6 feed) when I transmit.
Before I build a Low Pass filter for the K3 (like K8CU design QST, Nov 2002 p 34-36), does anyone have similar RFI experience on 6M.
Or should I focus on High Pass filters on the home antennas (what a pain).
My wife loves CBC Classical on 100.5 MHz...almost perfect 2nd harmonic of FT8 on 50.313!
Any suggestions from this amazing group would be appreciated to restore my marital bliss!
73 Jeff VE3CV


Bob McGraw - K4TAX
 

My TV OTA antenna is about 10 ft below my 6M yagi on the same tower.  No issues here even at 500 watts.  I suggest you determine if it is simply signal overload causing the issue or indeed 2nd harmonic.    Even if the 2nd harmonic of the radio is within specs, the signal overload may be the issue.   If so, add a 1/4 electrical wavelength of coax at the input of each of the receivers.   That will reduce the RF overload issue.

Using RG-6 with a velocity factor of 0.83 and a frequency of 50.1 MHz, the 1/4 electrical wavelength is 49.0" or 1.245 meters.  The open end does not have the center conductor connected to the shield.

See: http://www.arcticpeak.com/antennapages/quaterwavestub.htm


73

Bob, K4TAX


Tom Clifton <kc0vsj@...>
 

Would also ask if there is a common mode choke on the coax for the yagi.  May not be required, but can't hurt


Jeff Wilson
 

Hi Bob
Thanks for great suggestion.  Signal overload is probably the culprit, though I can see 100.5 MHz harmonic on my in shack SDRPlay2 SDR fed with a miniwhip antenna that is about 20feet from the 6m yagi.  I tried adding a choke at the yagi feedpoint which reduced sensitivity of my in shack RG-6 fed Harmon-Kardon FM receiver from 10W up to 30W output, but still bad above that.  I will use this RX to prune the stub to max 100.5 MHz.  Glad to have a use for all the RG-6 patch cables from years past.
I am also looking at putting highpass 54MHz filter on the house antenna before the signal amp, that feeds the home TVs and FM stereo receiver.
I expect there is a lowpass filter for 6m and below in the K3 TX must be pretty good.
73
Jeff VE3CV


Bob McGraw - K4TAX
 

A 1/4 electrical wavelength of RG-6 which has a velocity factor of 0.83 is 0.619 meters {24.37"} for 100.5 MHz
http://www.arcticpeak.com/antennapages/quaterwavestub.htm

This 100.5 MHz stub should be located on the 6 meter feedline, preferably close to the transmitter output to attenuate the second harmonic.  The 50 MHz 1/4 wave stub should be located on the input to the signal amp that feeds the home TV's and FM receiver.

The link I provided should allow the calculation of any needed frequency and considers the velocity factor of coax.  Do recall that 1/4 wave stubs are open while 1/2 wave stubs are shorted.  

73
Bob, K4TAX




Keith_WE6R
 

I suggest a 1/2 wave SHORTED stub instead. Just put a T in line like the link shows. It is easier to tune; calculate for the velocity factor, then just shove a pin into the coax and short it. You can easily move along to find the right spot. 
I use signal generator and a spectrum analyzer to watch and get the exact spot, then cut and solder.
The better the coax, the higher the Q. IE not as broad, and more attenuation. 
I have used this also on receivers to notch a FM or TV station that was over loading. 
Keith WE6R

 

 


Robert W5AJ
 

wouldn’t use the published velocity factor….

 

Testing method for cutting coax:

short end of coax for quarter wave – a good short, no inductance…

Transmitter or antenna analyzer to Tee connector  - then another connection on tee to 52ohm load, one to coax being cut

 

Cut LONG 2% to 3%

 

the short will be seen as infinite impedance at ¼ wave length (and you’ll only see the 52 ohm load)

 

The SWR will rise sharply on each side of line resonant frequency

You can find frequency line is resonant at and then cut until matches frequency you desire

excellent success with this method

 

I use the MFJ antenna analyzer as transmitter.

Coax stub method is fairly sharp on resonance, tuning is best way. 

Found this method in ON4UN’s low band DXing book and used it ever since.

 

 

73 W5AJ

 

 

 

Robert

Midland, Texas

 

From: Elecraft-K3@groups.io [mailto:Elecraft-K3@groups.io] On Behalf Of Bob McGraw - K4TAX
Sent: Saturday, June 13, 2020 8:40 AM
To: Elecraft-K3@groups.io
Subject: Re: [Elecraft-K3] 6 Meter Harmonics Problem

 

A 1/4 electrical wavelength of RG-6 which has a velocity factor of 0.83 is 0.619 meters {24.37"} for 100.5 MHz

http://www.arcticpeak.com/antennapages/quaterwavestub.htm

This 100.5 MHz stub should be located on the 6 meter feedline, preferably close to the transmitter output to attenuate the second harmonic.  The 50 MHz 1/4 wave stub should be located on the input to the signal amp that feeds the home TV's and FM receiver.

The link I provided should allow the calculation of any needed frequency and considers the velocity factor of coax.  Do recall that 1/4 wave stubs are open while 1/2 wave stubs are shorted.  

73
Bob, K4TAX