Low Pass Filter reducing forward power

[halfwave]

Hi again,

I have just noticed that my TX seems to be pushing out
reduced power while I have a Low Pass Filter connected.

A year or so ago I bought a Low Pass Filter off of Ebay
as did not have time to look into build one.

http://www.ebay.co.uk/itm/FMLP1WT-FM-Lo ... 54065f360a

which I have hooked up inline between my TX and my aerial.

If I take it off and measure the power I get the full correct
power, but when I put it back the power drops i.e.

With the Low Pass Filter connected my TX states 7 watts, but when measured
with my power meter I get 4.2 watts.

Will this always happen when a low pass filter is connected?
Or does it sound like there is a problem with the filter.

[s2000]

You normally get an insertion loss when using a low pass filter but usually the loss is quite low, seems you are losing a lot of power there.

When you build them from scratch you can tweak the coils to maximise the attenuation at the required harmonics and lower the insertion loss in the filter using a spectrum analyser with a tracking generator etc.. On a ready made unit I would expect that to all be done by the manufacturer.

Are you using 50ohm coax? All connectors 50 ohms?

[halfwave]

Yeah everything is 50ohm.
I guess there must be something gone awry in it.

I've taken it off now anyway as want the TX to transmit at full power.

Just a thought, if the TX has an built-in Low Pass Filter and I have another
one hooked up externally could that cause problems? or does it just double filter,
and would not cause a problem.

[s2000]

I would imagine it would filter the signal even more, but if the transmitter already has a low pass and the harmonics coming out of it are at a low level (no interference to other services), there would be no need for the external filter.

Have you had a look inside the transmitter? The low pass filter would be some coils and capacitors in a 'pi' formation (π) located right next to the place where the rf output connector is. Send a pic if not sure of the insides..

What transmitter is it?

[Albert H]

Looking at that Ramsey filter, it's really not going to work too well. The coils are all in line with each other - even with screens between them it's not a good idea. The capacitors are cheap surface-mounted ceramics, and the groundplane is too close to the coils. The coils have too many turns for Band II from a quick look, and the wire is much too thin. I find it hard to believe that a company like Ramsey could turn out something that bad!

Building a lowpass filter is really easy. You really do need staggered coils to prevent mutual coupling (both magnetic and electrostatic) and the capacitors should (at the very least) be silvered mica types. A quick test of a three-coil filter here just showed 0.15dB insertion loss at 12 Watts in. The second harmonic was reduced by a further 17dB and the higher harmonics disappeared altogether!

Take a look at https://ludens.cl/Electron/fmtx/fmtx.html. The whole design of his transmitter is pretty good, though I'm not sure about the need for all the interstage screens in the exciter. Further down the page, he describes the construction of home-made mica filter capacitors. His method is similar to mine, though I dip my completed capacitors in an epoxy resin to prevent them from corroding - heated copper tarnishes quickly in damp air!

[halfwave]

S2000,
My transmitter is a CZE-15B (I recently got it as I wanted one with variable power output so I could get it to cover exactly the area I want to hit and not any further).

Albert,
Thanks for the info and the link I will have a look properly when I get a bit of free time next week.

Sent from my Nexus 5 using Tapatalk

[s2000]

cze.jpg

Hi I had a look online, yes it does have a low pass filter (marked with the arrow), unfortunately (as Albert H mentioned) the coils are all inline and not at 90 degrees so its possible there may be some coupling between them. It would be interesting to see the output under a spectrum analyser to see if it does meet the manufacturers quoted ≤-60dB on harmonics.

[halfwave]

S2000,
Thanks for finding that image. At least I now know for sure it has a low pass filter. I don't have a spectrum analyser so unfortunately I won't be able to test to see how close it comes to the manufacturers specification.

On another note what is 'best practice' on how close to another occupied frequency you should be? I read somewhere that you should be at least 0.2mhz away either side of your frequency. So for example if you had stations on 93.4mhz and 94mhz, then would you be OK on 93.7?
Just want to check as my frequency is sandwiched in between two legals similar to the situation in the example above.
Obviously when I broadcast and am in the immediate vicinity of my aerial my signal wipe's out the legals. So I guess it also interferes with my neighbours reception of those stations, if they listen to them. So wondering if I should try and find another frequency with a larger separation, which may tricky.

Sent from my Nexus 5 using Tapatalk

[radionortheast]

that would be 300khz 0.3 away, you might be ok if the signals are really strong, we don’t know that and we don’t know what your nieghtbours equipment is or what their listening too, if their listening on a shower radio or an inferior radio with bad selectivity they won’t beable to hear those stations. But then again my parents listen to dab, their golden oldies stations is there, alot of people listen to online radio there isn’t forced to be anyone there listening to fm, guess its a calculated risk, better to operate out of way of other stations if you can.

i’d be more worried if the owners of the radio stations flick onto your signal, if those stations are based in your town if your transmitting right next to them…don’t suppose they’d like it

[halfwave]

Radionortheast,

Thanks for confirming the correct separation. I've had a look on the dial
and where I am is about the best option for me as else where other stations
are closer to my frequency.

I was scanning the dial on my way to work and found another station +0.1Mhz away from
me but it was in some foreign language, (couldn't work out what language it was). Also
it did not appear on http://fmscan.org/ so guess it must be a local 'non legal' station to
where I work. Luckily it is far enough away from my TX that we should not interfere.

I will have to test on the way home from work and see how far it's signal propagates.