Broadcast AM filter - RF power tolerance?

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tustinfarm
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Joined: Wed Jan 29, 2020 12:18 am

Broadcast AM filter - RF power tolerance?

Post by tustinfarm » Wed Jan 29, 2020 12:35 am

I have one of the "official" RTL-SDR.com broadcast band filters, which really helps at my location, since there is a 1340 KHz tower just a half a mile away. Without the filter that AM station shows up all over the place.

I would like to use the same filter with a low power (1 watt) HF transceiver (running at 7 MHz) that I am experimenting with. It also suffers from overload by the local AM station. Does anyone on this forum have any idea if a filter of this type would tolerate a full watt of RF energy being transmitted through it? Since the filter does pass the 7 MHz frequency with minimal loss, I would not be "asking" it to dissipate much energy, of course. I may eventually get the courage up see what happens, could always buy another one if I fry it.

alanzfq
Posts: 76
Joined: Thu Oct 04, 2018 11:18 am

Re: Broadcast AM filter - RF power tolerance?

Post by alanzfq » Wed Jan 29, 2020 12:49 pm

Looking at the product page I see it has small SMD inductors.
I'd think those and maybe the capacitors would be stressed by even one watt.
For a TX I'd suggest something designed for TX, like the QRP Labs LPFs.
Alan

alanzfq
Posts: 76
Joined: Thu Oct 04, 2018 11:18 am

Re: Broadcast AM filter - RF power tolerance?

Post by alanzfq » Wed Jan 29, 2020 12:52 pm

Sorry it is a HPF but I think still needs to be more substantial.
There are design tools on the web.
If not an answer could be to insert it after the TX/RX changeover.
Alan

qrp
Posts: 44
Joined: Wed May 22, 2019 11:16 pm

Re: Broadcast AM filter - RF power tolerance?

Post by qrp » Sun Feb 02, 2020 3:13 pm

it depends on how much power will be consumed on filter components due to heat loss, so technicaly it depends on the filter component quality. Better inductors and capacitors Q-factor, leads to smaller heat loss.

Also it depends on capacitors maximum voltage. Usually SMD capacitors have 25-50 V limit. You can use LTspice to simulate filter and look at maximum voltage applied to capacitors at specific input power level.

Technically, if it is built from high Q-factor SMD components, it may bypass 1 W power with no issue. But don't apply it for a long time, to avoid overheat and damage.

rtlsdrblog
Site Admin
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Joined: Mon Nov 19, 2012 11:54 pm

Re: Broadcast AM filter - RF power tolerance?

Post by rtlsdrblog » Thu Feb 06, 2020 12:43 am

We only designed it for RX, so I can't confidently say what will happen if you TX through it. If you decide to risk it and try it, and it's fine please let us know your results!

nospam
Posts: 18
Joined: Tue Jan 03, 2017 10:08 pm

Re: Broadcast AM filter - RF power tolerance?

Post by nospam » Fri Feb 07, 2020 6:16 pm

I would use a relay to bypass around that filter. Simple thing to do. Most xmtrs have a signal that comes out that asserts when you transmit. I made one for my rsp1 and my ham rig. When I xmit on the ham rig the antenna switches to the rig and another set of contacts grounds the input of the rsp1 just for extra safety. Besides the power question of the BC filter I'd be concerned about what the impedance would be, maybe its 50ohm but when you start putting power thru things the ferrites can saturate and things don't act normally.

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