What is a filter?

[kmc127]

Hi All-

I am just about to buy a 817ND but I saw it has filters you can buy for it. Can anyone explain to a new ham why you would need these filters?

Thanks!

[m0bov]

Narrow SSB filters can help remove adjacent QSOs on a busy band, DSP can help with removing static and white noise. TBH an 817 on a decent antenna is going to get overloaded anyway, if you want filters and think you'll need them, go for a 706 or 7000,

[John_D]

In addition if you are going to transmit I suggest you get a "Low Pass Filter" with a cut off of 30MHz. This fits into the coax line just after your transmitter. It stops the harmonics from getting into you antenna and radiating into TV, radios, recorders etc. An essential part of you equipment. If you are going to use CW, then a narrow CW filter will cut out the adjacent QRM to the station you are working.
Broadly speaking, filters will make the received signal narrower.

[5B4AJB]

Most radios have a provision to fit 2 or 3 filters which vary in bandwith according to the modes you wish to work.

Narrow CW filtering is essential if you're serious about CW, a suitable one for RTTY is nice too.

Imagine a contest, you're trying to work someone on SSB, there are lots of strong stations either side.
You'd select a narrow filter to "clean up" the incoming signals by attenuating either side of the received frequency.

How much attenuation and how much the filter lets through depends on the mechanical properties of the filter selected, so, as before, a narrow CW filter maybe only lets 25Hz through either side of the tuned frequency.

A typical SSB signal is 2.3 to 2.7kHz wide, if your frontend was 3.5kHz wide, it'd sound more natural, but other adjacent stations may splatter over the station you are tuned to. If you selected 1.8kHz on your SSB signal, it would let less of the interfering stations in, but also less of the one you were trying to copy (also sounds a bit less natural, but it might enable a QSO where otherwise not).

[kmc127]

Ah, so since this is my first radio I don't think I need to invest in a filter yet. It is interesting as a new tech ham I can use ssb in 10m... maybe I will get an ssb filter some day =) I am guessing really good radios come with filters already?

[John_D]

No... you WILL need a Low Pass Filter, especially if using SSB. The modulation in SSB creates harmonics which need to be suppressed.

[m0bov]

I beg to differ, modern rigs should not be producing "sprogs" and fitting a LPF will mean switch box and more patch cables and loss etc.. etc.... I've never heard of anyone using these filters other than contesters so 5w should not prove an issue. I'd wait and see, the filter (good one) is likely to cost more than the antenna and larger than the rig!

[John_D]

so 5w should not prove an issue.

Where does 5 watts come from?
Quote from the FCC licence conditions:-

Unless otherwise noted, the maximum power output is 1500 watts PEP. Novice/Technicians are limited to 200 watts PEP on HF bands.

There are no switch boxes necessary for a low pass filter, it simply fits in series immediately after the TX. Losses are in the region of 0.5db.
Finally regardless even modern rigs manage only between -50 and -60dbs of attenuation with internal filtering.
Its better to be safe than sorry in this case. One case of RF breakthrough with a neighbours appliance could well ruin any activity.
Regarding price. why will cost more than an antenna-? The average on Ebay is around £30
You also question the size of the filter being as large os the rig..... I am beginning to think we are talking about two entirely different items, for example the Yaesu FF-501 is no more than 9 inches long and 1.5 square.

[M0TTB]

Where does 5 watts come from?

It's the maximum output power of the radio in question.

There are no switch boxes necessary for a low pass filter, it simply fits in series immediately after the TX. Losses are in the region of 0.5db.

Yep, I agree, losses not worth bothering with at HF/VHF

Finally regardless even modern rigs manage only between -50 and -60dbs of attenuation with internal filtering.
Its better to be safe than sorry in this case. One case of RF breakthrough with a neighbours appliance could well ruin any activity.

An external LPF is not a vital requirement with a modern HF radio as you make out. Sales of these items have plumetted over the last 20yrs for good reason, modern radios are cleaner and domestic potential 'victim' equipment far more robust. For such a miniscule chance of causing a problem, that would be cured using an external LPF, it simply doesn't make sense to buy one from the outset.

You also question the size of the filter being as large os the rig..... I am beginning to think we are talking about two entirely different items, for example the Yaesu FF-501 is no more than 9 inches long and 1.5 square.

maybe have a look at the FT-817... they really are tiny, maybe not quite as small as a compact LPF but not far off.

[m0bov]

The 817 puts out tx from all the bands on one connector, so the user would need some sort of duplexer or switch and it just sort of ruins an otherwise compact and neat setup. My view anyway.

[John_D]

Where does 5 watts come from?

It's the maximum output power of the radio in question.

Now THAT I did not know... so my apologies (BLUSH) I foolishly assumed it the power was the same as other HF.VHF.. rigs... (runs to hide under bed)

Ok, we agree to disagree about LPFs... but with QRP, I would agree they are unnecessary...