Coaxal Notch Filter for 70cm you can build for almost nothing!
Cowles Andrus III K4EME
Hi All,
During the first leg of the EME contest, I had an issue with some TVI on our TV when I was in transmit mode on the 70 cm band.
This is very understandable considering the small distance between my EME array and the TV antenna.
It was simple due to the TV booster overloading during transmit.
The distance between the EME array and the TV antenna is less than 100 feet, and at close to the same height.
On top of that, the TV antenna is aimed toward the EME array for most of the local stations.
Every time I would transmit I would completely wipe out the TV and my XYL's movie she was watching.
We did not much problems with this in the past until I decided to drop Dish / Direct TV due to the cost exceeding $100 per month and off the air signals are free.
Since we are high on a hill, we get 70 plus off the air stations.
I did loose a couple about a year ago until I installed a LTE filter to block the new LTE signals from the nearby cell tower.
The LTE filter cost less than $7 shipped from eBay, so well worth the investment.
Looking for a 432 MHz notch filter, I found a couple, but they were in the $100 price area.
I needed something that would attenuate my 432 MHz signal without much insertion loss at the UHF TV frequencies.
I wanted to install the filter between the antenna and the TV booster or preamp so low insertion loss was necessary not to affect the system's overall noise figure.
I was at the dump on Saturday still cleaning out my Mom's house so it would show better for selling it.
I saw two TV's and some RG-59 coax jumpers being thrown away that someone set out for grabs.
I kind of remember building a 1/4 wave open notch filter back in my college days to remove an interfering carrier.
The coax must have triggered the thought about constructing a notch filter.
I pick up the coax and took it home, it appeared to be in new condition.
I remembered that to form a notch filter, you can use a piece of coax 1/4 wavelength long, open or non-terminated.
The formula is simply : (C) the speed of light/(f) frequency = (╗) wavelength
I wanted the wavelength in inches and knew the speed of light in miles per second.
(186,282 Miles / Sec) X (5280 Feet /Mile) X 12 Inches / foot) = Speed of light in inches / second
C= 1.1803 x 10^10 Inches per second
C / f = 1.1803 x 10^10 Inches per second/432.050 x 10^6
Wavelength = 27.3214 inches
Since we are after ¼ wavelength and remember the speed of light changes in coax by the velocity factor you must multiply the wavelength by both of these to come up with the proper stub length.
(Wavelength) 27.3214 inches X (¼ Wavelength)0.25 x (velocity factor of RG-59) 0.66
Length of stub is 4.508 inches.
The actual length was almost dead on with it's calculated length.
Below is what the trap or notch filter look like when sweeped:
http://cowlesradio.webs.com/432MHz_0.25%20%20Wave%20open%20stub%20coax%20Trap.JPG
The only thing that is needed besides a small jumper and weather proofing is a 3 port female f-type connector.
There is no reason why this type of notch filter would not work well if designed for other frequencies if needed.
It provides over 25 dBs of signal rejection at it's notch frequency, 432MHz, while having almost no insertion loss at 470 MHz, the low end of the UHF TV band.
It does have reentering notches at multiples of the primary notch frequency, however for 432MHz x 2 = 864 MHz which is above the top UHF channel.
Before spending a lot of money buying a notch filter, you my want to try to build one from your scrap box.
Best regards,
Cowles K4EME
432 MHz ExtrEMEly Low Noise Preamp for sale: http://cowlesradio.webs.com/drrf.html
Neil Buckingham
--
You received this message because you are subscribed to the Google
Groups "ValleyHams" group.
To post to this group, send email to valle...@googlegroups.com
To unsubscribe from this group, send email to
valleyhams+...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/valleyhams?hl=en
Please do not send jokes and junk e-mail on this list - Let's use it for our various hobbies, public service, communications and any good useful information for our hobbies and Events.
---
You received this message because you are subscribed to the Google Groups "Valley Hams" group.
To unsubscribe from this group and stop receiving emails from it, send an email to valleyhams+...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.