Low pass filters

[jrd426]

Hi, is there a low pass filter made that keeps transmissions at 27/28 mhz or is something like that not possible? I think most i have seen are wide keeping things below 30mhz. Reason i ask is that i have watched a few vids by Scotts where he is using low pass filters when using rm italy or similar amps to deal with unwanted harmonics. Thx fellas.

 

[brandon7861]

Low pass filters are wide by design, as they pass anything below the designed frequency. For eliminating harmonics, thats just fine as the harmonics start at 2x the operating frequency (54MHz) and go up from there. You will not have harmonics below 27MHz. You can have spurious emissions below 54MHz come out of a radio, but if bad enough to be a concern, that is usually a sign of something else messed up under the hood and an output filter is not the way to handle it.

The reason most low pass filters allow up to 30MHz is because they start to roll off at the design frequency, so its best to design them for a little higher 3dB cutoff frequency.

It sounds like what you want is a bandpass filter centered at 27MHz, but I don't see how using one would be of any benefit.

 

[jrd426]

Thanks for the info Brandon. I ask this question as i have a amateur neighbour less then a mile away from me and was thinking it would be good to keep my 27mhz activity out of his shack if possible. Yes a bandpass filter is i think more what i was aiming at.
Thx and 73's

 

[brandon7861]

Start with some logical assessments of his hobby. Is he a tech or a general/advanced? What kind of antennas do you see at his place?

If he is a technician and only has small antennas, I would assume he doesn't even know you are on the air. Assume you are not interfering with him and call it a day. If he is a general or advanced and has a yard full of antennas, theres a good chance he has something with a waterfall display and sees every signal that comes out of your radio.

If he does do HF, why not go introduce yourself? There is a decent chance that he can tell you if you are transmitting out of band or not, and if so, may even be able to help resolve the issue. Not all hams are stuck up CB haters. Some are, but not all. I like to hope that most would rather help someone solve an interference problem.

If you are emitting a signal where it shouldn't be, adding a filter to stop it from getting out doesn't stop your radio from producing it. Its still wasted power and added heat in your finals.

 

[jrd426]

Some good points Brandon. Since i have moved to my present location and knowing hes basically around the corner i have done very little TX'ing not wanting to piss him off even using a stock cb 4/12w setup. I guess at the end of the day its compromise and respect.
Thx for your input.

 

[guitar_199]

One thing I feel should be thrown in and @everybody!) if I am wrong... let me know it!!!!!

But if you are a CB operator then your "fundamental" frequency is in the area of the 11m band/27 Mhz. Harmonics will only be multiples of that fundamental frequency. You won't be generating any harmonic content lower than 27 Mhz.... unless your radio is SERIOUSLY leaking some lower oscillator content out through the finals. With this in mind, a low pass still seems like it could be in order... as it would knock down the harmonics above the "fundamental frequency" of your transmitter(s).

Where I have seen bandpass filters used is in "field ops" or "contests" where you have multiple operators... on multiple bands... working in close proximity. That helps to keep them "out of each others knickers" so they can work the bands at the same time...( still.... one working a given band at a time please!!! )

A low pass should do what you want..... UNLESS you are so close that your fundamental is overloading his receiver..... in which case..... give HIM a bandpass filter as a "peace offering" !!!!!!

 

[jrd426]

I appreciate all the comments. Well i have this here that i have been lugging around for years and probably should just use it seeing i have it.
Thx 73's

 

[Xracerx13]

I have that same exact one. Sitting in the closet since the 90's. Its one of the better ones.

 

[sp5it]

If your radio is not butchered, no overdriven splatterbox inline, there is nothing to be worry about. Just enjoy radio.

 

[dave457]

Bandpass filters for whatever band you need, I have one for 11 meters only, starts dropping off below and above the standard 40 ch.s. I'm sure one can be bought and tuned for 27/28 mhz

 

[Crawdad]

Some good points Brandon. Since i have moved to my present location and knowing hes basically around the corner i have done very little TX'ing not wanting to piss him off even using a stock cb 4/12w setup. I guess at the end of the day its compromise and respect.
Thx for your input.

If it was me, I'd go knock on his door, introduce myself and explain the situation. My bet is you'll find him to be helpful and welcoming to you. He may even give you the Why-not-get-your-amateur-license sales pitch. He may also be a great source of info/knowledge (aka an Elmer).
If he's into HF, then he likely has a transceiver with good Selectivity, meaning he'll probably never hear you with or without your amp.

7 3 and have fun

 

[Crawdad]

i have done very little TX'ing not wanting to piss him off even using a stock cb 4/12w

And I should have added kudos for being respectful of others in the hobby. A rare commodity these days.

7 3 again

 

[jrd426]

Some good info and suggestions. Thanks. I think ill dust off the filter i have kicking around here and see how it changes my current swr if any. Im curious

 

[dave457]

Put it between the radio and amp

 

[jrd426]

I would have thought it would need to go behind the amp if running one. To clean up a likely splatter box as someone mentioned earlier which in my case would be something like a small rm203 or similar built unit. What got me on this subject was watching 2 vids from scotts radios where hes using the low pass filter (drake i think) after the rm italy amp in both vids to clean up the final output signal. I will do a swr test in both positions just for the halibut.