The Filter PSA: Or why is my noise floor bouncing up and own?

Hi everyone!

Events at my shack this weekend have reminded me of a very important rule involving SDR receivers, one that doesn’t get talked about much and is frequently overlooked by newbies to the sport: external RF filtering. Specifically receive filtering, TX filtering is a different subject though just as important for the SDR user as these little babies signals aren’t the cleanest out there.

We have had some exceptionally good conditions here in the southern UK for the last week, I’ve been having great contacts from 160 all the way up to 6M (I think we are in for some usability there again, finally!), lots of DX and the SWL listening has been phenomenal.

Well, mostly. This brings me to the subject of this post, filters.

Because conditions are so good right now one of my neighboring Ham operators a couple of streets over is very active just like I am. Neither of us is using anything special, standard folded dipole stuff way too close to the ground and around 100W output when we TX but I have a problem that he doesn’t: I am using a fully SDR station right now.
LimeSDR and old school Atlas based HPSDR to be exact.

They both have the same problem when in the vicinity of another transmitter; they have wide open front ends that are easily swamped by out of band signals.
The LimeSDR is particularly susceptible to this kind of interference because its front end is taking in 60MHZ of the spectrum in one big gulp so any signal in that range causes the noise floor to rise accordingly. This means all those lovely DX signals you were hearing seconds ago are now a wash of static until your neighbor stops transmitting.

But what about the internal filters a lot of you will say?

They are what they say are, they are -internal-. The front end A/D convertor is still swamped and the noise floor still rises because of it ruining what was a perfect day of DX for even the most stubborn of us.
And it’s not just local hams, I get something from one of our local industrial parks that sweeps across VHF bands as well making Marine listening impossible while it is active, and I only live 1/2 a mile from the Solent!

So what do we do if the internal filters don’t help this stuff?
We use external filters of course!

These are the little boxes you can buy or make that are known as Band Pass Filters.
They do what the name on the tin says, they are filters that have both a high pass and a low pass filter arranged in such a way that only a particular band will get to that very sensitive SDR A/D front end.
They are placed in line with the antenna and they attenuate signals outside their specified pass band,
so for example a 40M Band Pass Filter would attenuate signals below 7MHz and above 7.3MHz while allowing the maximum signal through inside the pass band. That means for you that if the neighbor is working 80M or 30M you can happily DX on 40M to your heart’s content, at least until another OM pops up on your band nearby.
These filters can be had for SWL as well, this is great if you are an avid SW listener during these great conditions and I or one of my fellow Ham types decides to start working the bands, you just pop in the filter for the band you are listening to and we disappear like magic and your listening pleasure increases to the point that you start to wonder why it took you so long to get one of these little gadgets!
You can get them for every Ham band from 160M up to 23cm, most Marine/Air and commercial bands, SWL and scanner bands too!

But Todd, what if I am contesting with my SDR and NEED to be on the same band as my neighbor?
Great question, good thing for you I talk to myself…

In these cases what you need is an adjustable Notch Filter.
Not so easy to find, but not too difficult to make either, I recommend hitting the ARRL Handbook for some ideas.
These little babies are the bee’s knees for making that abomination trying to ruin your contest go away.
They work the opposite manner to a band pass; they create a sharp notch that attenuates only the signal it is tuned to. Just watch the waterfall and tune the filter, you will see the notch moving around as a dip in the noise floor. Tune it right over the offending station and the band is yours again!
You would use one of these WITH a band pass filter; you still want to be filtering out the outside noise.

But Todd! It’s a field day! If I hook up 8 or 9 of these to the front end it’ll be dead from insertion losses, what do I do?

Well, remember the band –pass- filter?
What you want is not a band pass but a narrow pass filter. It is like a notch filter in reverse.
In this case you tune it to the frequency YOU are working and it blocks out everyone on either side.
Now it doesn’t matter how many Hams are pounding away on the keys or mics in your band, you just shut them out. Now do keep in mind we are now talking about some high Q filtering here, you are sitting in a tent with 4 to 10 other guys some of them running up to 500W into beams that are within 30M of yours so your typical band pass filter design will not cut it. Cavity filters are the best but they are hard to tune and really big at these frequencies so a good way is to stack a couple of good quality sheilded narrow pass filters with a good LNA between them to take care of some of the losses. Don’t go putting the LNA on the front end though; you’ll just amplify those big signals into that first filter making it hard to attenuate them out all the way.
I have one I made that allows me switch out the LNA and switch between the filters so I can tune them to the frequency I want then switch it all back in for use. Works great unless that big guns beam sweeps my antenna, in which case no one is happy including the guy with the £5000 analogue rig crammed pack with all the filters I was just talking about when it happens to him.

But one of the best filters I have ever run across for field day, the one that kills everyone else around you right at the antenna is the venerable old ‘we still aren’t exactly sure exactly how this thing works as a TX antenna’ Magnetic Loop TX antenna. It’s an antenna with such a sharp tuning range that it effectively acts as a receiving and transmitting filter as well. If you’ve never tried one give it a go, it’s the ultimate field day antenna. It not only likes to be close to the gournd it NEEDS to be close to the ground to work properly, it wipes out signals outside it’s tuning range like magic, it can be oriented so there is a null on any wide band noise (or that big guns big beam), it is small and transportable and most of all it just works. It’s no 6 element beam or single band dipole at 60 feet but we are talking field day here.
Check em out, they rock a small garden and make the noise disappear before you even add filtering.

In short filters are great, give them all a try. You really won’t regret it.
Best thing you could do for your SDR aside from shielding it.
Go pick yourself up the entire set and have more fun.

See on the air!

Todd M0GLO


everything in this world seems to remind me to find a bandstop filter for tetra band but so far no luck

If any of you have old ham radios, dremel out the board that had the bandpass filters & control system. You can do that with the LPFs & amps, too.
I will be using an FT857D PA/filter board with mine. All ham bands 160M-70cm (Excepting 1.25cm).
I was going to use a TS-440, but adding 2M & 70cm amps, BPFs, LPFs, directional couplers, attenuators, relays…
All of a sudden, I didn’t have enough room.
Found the FT-857D chassis with burned finals for $125 on Ebay. It’s a convoluted, but pretty good control system. 10 minutes with the schematic & I saw that it was perfect for the job. Now, I’m adding a couple dongles & other stuff.

Find a local ham swap, look for an old 80s junk ham rig. Beat them up on price (It’s a ham thing). Many late 80s into 90s (TS-440 for example) have continuous coverage receivers & 12-16 BPFs in the receive chain. You get to cover the whole HF spectrum.


oh2eko A bandstop filter can be made with a Tee coax connector & a length of coax. I am not sure of the equation, but am sure there are calculators out there.

M0GLO Excellent write up. Love the loops. Some day, I’ll finish my 4 nested loop antenna. Outer loop is thick walled 2" dia with 7’6" diameter. 3 progressively smaller loops on vertical square tube & relay switched ferrite transformers. All welded aluminum. 15" long 2 gang capacitor with 6" plates & 1.4" spacing. Square tubing (With spacers & plate) will be welded to the actual stationary plate mounts. Luckily, I work with some of the most talented Tiggers I have seen. The loops were hanging on the wall at the shop for 18 years. Now, they will get used.
I am using Loftur’s controller for a 2 turn 7/8" heliax loop now. It’s great on 40M to80M.


coax stub filter is wayyy to wide i did try that but as i only want 5Mhz blocked out (390.0125 - 394.9875 MHz) end result was pretty nasty.
Foor good rx operation here i would need BFM Tetra and a lowpass for around <—500Mhz.
I happen to have a tetra/gsm/4G/maritimevhf/ tower right next to me.

I know that the TV transmitters have adjustable low pass filters that should do just fine. . I will see what reject & BPFs they have. The frequencies you mention are right in our land mobile range (USA FCC allocation).
There are several other uses of that area for frequencies that may be tunable for your reject filter in the USA. Check this chart (Long)
Now, when there is a commercial transmitter farm (A few to hundreds of transmitters per site) ALL must have specific traps & filters, so as to not create intermod distortion, harmonics & specifically stop many frequencied that are more important than a TV transmitter. These need to be tunable, to adapt to each site.
I’ll see what is in my pile of stuff here. Maybe you can find similar where you are.


Well, I must have another box somewhere, but I did find close to this (UHF only) The part number is the same, except for the G3 is a G1. Should do well for a low pass filter.
There is also a 3 plunger bandpass filter & a single cavity (Cannot find at the moment, that I think is a notch filter).
These are in an old 80s - 90s Larcan 40W transmitter.
On the chart in my previous post, you will also see that 400-430 Mhz is used for meteorological & sattelite/space communications. There must be filters for them, too, that may be tunable to where you want.

This series of older US military surplus filters are adjustable & can be altered for different ranges. I “Think” www.repeater has instructions for altering them to properly suit ham bands.

If you can find a local electronics surplus liquidator, you can probably get similar parts, after opening units & checking part numbers.

Yes, I am a cheap ham. I buy junk to make better junk. Can’t have too much junk.


You know, given that you are getting hit by signals across a number of bands the proper strategy is to use band pass filters only for where you want to listen.
It is very very hard to filter out several high level bands of signals like Tetra, you are talking 6 or 7 bands from 300MHz to 1200MHz and that kind of filtering for individual bands means stacking filters until you have degraded the signals you want to hear into the dirt. It’s a lot like having a dozen hams in the local neighbourhood all working different bands than the one you want, just block them all out while passing your band through is the easiest, lowest loss solution.

Just pick up several band pass filters for where you want to listen, they’ll attenuate the Tetra and other noise right out. Switch them in one at a time as you move around your regular listening bands and you’ll be good.
Well, most of it anway, you might need a notch filter or two to catch the harmonics and mixing products from those towers as well.

I’ve got a 4.138 foot diameter loop of thick walled 2" sweat solderd that gets me from 60M to 40M with a 15kV 0-150mf Russian vac cap and gamma match. Good for 100W and portable.
Gotta remember though the near field EM on these things is murder, keep the kids and animals a good distance from them when transmitting! I always cringe when I hear of hams using them inside their shacks, seems a lot of us immediately forget what we tested on once we’ve passed.

Been thinking about getting it cleaned up and properly TIG welded, won’t make much difference really on the R of the loop but it would sure be nice to look at.
Let us know how the homemade leaf cap works out for you, I’ve seen similar on a couple of commercial designs and always wanted to know how well they worked at higher inputs.