the funny thing is that low pass filters operate bi-directionally. If your radio produces harmonics that interfere above its normal operating range, then mix down products from sources above HF would also cause interference on HF. THese mix down products are often caused by interactions within the first stage of the IF in your radio. SO, using a filter eliminates a lot of that from getting to your radio (thus reducing or eliminating the problem).
I have an FM radio station transmitter site less than 3 miles from me producing several thousand watts. without the filter, I hear what sounds like FM modulated signals in a couple of bands. I put the filter in place, and those signals disappeared. THere are other sources (such as a wireless Access point, LAN switches and cable modems'DSL modems that produce signals that can show up in HF directly. Filtering these is a bit harder. I am having to use ferrite beads on the power leads and also on the LAN cables to try and minimize some of these issues. I have only been partially successful. I still get a lot of birdies in the 10 meter band (around 28.5). I am going to acquire a portable shortwave receiver and try to track these signals to their source and see what I can do to remove them.
-eric
On Feb 1, 2014, at 4:37 PM, John Miller wrote:
> I've never had a low pass filter help on receive. I haven't run one in a
> long time though.
> ----- Original Message -----
> From: "eric oyen" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Saturday, February 01, 2014 3:21 PM
> Subject: Re: Low Pass Filter
>
>
>> one would think so, however, there are a lot of other devices that =
>> produce noise that can be mixed down into the HF bands (either by =
>> interstage mixing or inter mod).. a low pass filter is still useful for =
>> reducing that incidental noise to acceptable levels. Also, when it comes =
>> to a phone patch, those are still useful (as cell phones don't work in =
>> all places).
>>
>> I read this somewhere: "don't forget what you already know".
>>
>> -eric
>>
>> On Jan 31, 2014, at 11:31 PM, Howard Kaufman wrote:
>>
>>> With modern radios and modern television, their is little need for a =
>> low=20
>>> pass filter.
>>> If you are going to use one, it should be as close to the transmit =
>> output as=20
>>> possible. With so few stations transmitting from channels 2 through =
>> 6, and=20
>>> the modern rigs having both auto tune and internal tuner features, I =
>> think=20
>>> the low pass filter has gone the way of the phone patch, and the
>>> plate=20=
>>
>>> modulator.
>>> =20
>>> =20
>>> ---
>>> This email is free from viruses and malware because avast! Antivirus =
>> protection is active.
>>> http://www.avast.com
|
