LPF in active antenna
Also what about the 1uF input capacitor?
The filter needs to "block" a maximum of 5W of RF power from reaching the preamplifier.
those 3 x 1uF caps in the RF lines are way too big a value to be used at RF freq's
That value is suitable for audio freqs
They should be down in the pF range
I have changed these to 100nF. They seem good down to 10-15KHz, then they start to attenuate, which is desirable.
What about the LPF components? This is the main concern.
Do they need to be of higher power (like silver mica caps), or normal SMD will do?
The filter needs to "block" a maximum of 5W of RF power from reaching the preamplifier.
Any news about power of components?
Here is a subjective way of looking at it. Picture how much current runs through the capacitor. There is a certain Amperage that it can tolerate. It might be in the leads, or in the plates (foil, whatever). It partially has to do with ESR. Probably also ability of internal parts to withstand heat.
It's typical for us to think about watts through a component. I'm not sure watts applies directly in this case. That's why I mention Amperes.
There are objective specs (datasheets) for some types of capacitors, probably ones used in demanding applications.
Sure, however I am talking in a more practical way. Would you feed 5W into such a filter made out of SMD 0806 components?
First of all I would redesign the low-pass. Presently it's shorting the transmitter at 144/433 MHz.
I do not want the signal to be shorted on 144/433.
I want it to be reflected so that the main antenna can transmit it.
If it is shorted it is dissipated I think?
Can you please suggest me the filter to use? (at least the topology)
The LPF should end with a series element (inductor) towards the antenna jack and not have parallel capacitors with the inductors.
It depends on which components really have 5W going through them. Did you design the LC filter to stop the 5W content? Then certain components do not have much current flowing through them. Tiny components will be safe in that position.
Your schematic has one or two 56pF capacitors placed so they pass high frequencies to ground. They present a certain impedance to your AC frequency. By calculating the impedance, it gives you an idea of the Ampere level they can possibly carry (depending on voltages in your circuit).
Your schematic has 1N4148 diodes in anti-parallel, placed to divert voltages over 0.6V. (Perhaps they are safety diodes?) It implies your circuit runs at less than that amplitude. So we ask, which components really carry 5W? 5W implies Ampere levels upwards of 8A. This sounds like a lot to handle. Look at the L or C reactive impedance to tell you how much Amperes it carries at the applied voltage.
So it has to be something like this, so that the input/output inductors can block RF instead of shunt it to the ground (so that no RF power is lost but instead reflected)?
Wrong image, this is the correct one.
Much better now.
Allright thanks very much, I learn something new!
I did not know the difference between series or parallel for the first element of the filter, obviously "reflecting" or "consuming" the signal is a major one.
Thanks!
The filter you proposed worked fine!
I actually put a 5-pole LPF of that type there, composed of three inductors.
I have not done any measurements, but the preamplifier or the protection diodes do not seem to be heated if fed with 5W of RF power at the connector, so I guess it attenuates ok.
After 60MHz or so, I can listen less noise on the receiver with the preamp turned on. On FM band, turning the preamp on or off has no difference in signal reception. So I guess the filter responds as designed, but it may have to do with the input ferrite bar as well.
To verify that the filter works ok, I measured the output power on the rubber TX antenna, by disconnecting it from the transceiver and feed the signal to a power meter and a dummy load. At TX 5W, I could detect about 5W on the power meter, so I guess the filter does good job in reflecting power towards the TX rubber antenna instead of consuming it?
Hi,
I built the LPF in post #12 but with one more pole, i.e 3 coils in series and two shunt capacitors to the ground at their joints.
The coils are 120nH-280nH-120nH and the shunt capacitors are 100pF.
I have used silver mika caps and T30-12 cores for the coils. The max input power is 5W at both UHF (433MHz) and VHF (144MHZ). This is the power that needs to be "reflected" back to the transmitting antenna as shown in post #1
Because space is critical in my application, I would like to see if I can buy SMD coils and capacitors for the purpose.
are they fine for an input power of 5W and what kind of coils/capacitors should I use (chip types?)?