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Need help/info about a low pass filter; Is it wise to connect one anyway?
Topic Started: Wednesday, 17. October 2012, 18:42 (1,471 Views)
Gert (19CT006)
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19 Charlie Tango 006

Hi all,

At this moment I'm very busy designing my new base station to become. I just moved to a new place and I have to start all over again after quite a few years of absence on 11 mtrs.

My very first choice in aerials would be the Sirio Gain Master because of it's performance and very low TVI. The only problem is that I'm not able (allowed) to mount it because of it's length. Therefore it's probably going to be a Solarcon (Antron) A99. I've owned three of them in the past and I must say that they've always performed very well, at least compared to what you could expect of a half wave GPA. Despite that, unfortunately this antenna is also known for the "bleeding" of harmonics, a.k.a. TVI....!!

That's why I'm considering to purchase a low pass filter. I've been browsing the internet and I came to two potential options:

- RF Limited DF 3000
- MFJ 704

Both cost (approx.) 70 Euro's here in The Netherlands.

I've never worked with a low pass filter so I surely can use some advice. Is it wise to connect one in the first place? I mean: everyone has digital TV over here, but of course there will always be transistor radios, PC speakers and other stuff that could pick up my ssb activities...

I know that I could also get rid of the problem by mounting an "ugly balun" but I really wouldn't know how to make one and (next to that) that would only make the harmonics disappear which are coming straight from the antenna. My coax will be about 30 meters (!) long and it's running straight through my appartment building, so that's why I think it would be better to make sure that a clean signal will be sent to the aerial, since the coax is a part of the aerial installation as well. (The cable will run through a rain pipe and passes some neighbours.)

Does anyone have any experience with one of these two filters? Or does anyone have some other suggestions?

Oh yeah: I will mainly be pushing original RF power (30 W), with the option to insert my Zetagi BV 130 (200W).

Thanks in advance for your comments and/or advices! :thumb:

19-CT-006 / Gert

P.S.: Excuse me for possible typographical or grammatical mistakes, but after all I'm Dutch right? ;)

19 CT 006
Operator GERT
Hillegom - The Netherlands

E-mail: [email protected]
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Persona non Grata
Wednesday, 17. October 2012, 18:42
I've been browsing the internet and I came to two potential options:
  • - RF Limited DF 3000
  • - MFJ 704
I've never worked with a low pass filter so I surely can use some advice. Is it wise to connect one in the first place?
I am not familiar with the RF DF 3000, but I have heard a number of people complaining about poor soldering on the MFJ 704. If you take than option then it would be worthwhile checking inside the case to ensure it has been put together properly.

Personally I have always used the Kenwood LF-30a or the Yaesu FF-501 which can usually be found secondhand for around 25 / 30

Is it wise to connect one? - I would say that the potential benefits outweigh the potential downside.

  • the Filter will significantly attenuate any signal coming from your radio above 30MHz, so if your set is "dirty" and sending a VHF Harmonic, the Filter wll mute it.
  • Using a Filter does provide some peace of mind, especially if operating in a potentially "hostile" location.
  • the Filter itself is an additional item to go wrong in-line with the aerial, so (if faulty) it could degrade the signal or potentially give zero connectivity and, obviously, if you transmit into a Filter with zero connectivity, you run the risk of damage to the radio.
  • using a Low-Pass Filter means adding a patch lead and two additional PL259s to the aerial line, that represents additional potential points of failure
  • those PL259 are "Lossey"
Personally, I do use a (Kenwood LF-30a) Filter on my own set-up although I am not aware that I have any TVI Problem. I find it worthwhile on the "Better to be safe than sorry" theory.

BUT, a Low Pass Filter is not the Be-All-and-End-All of "Anti TVI" precautions, it is ONE weapon in the armoury. I would recommend that you:

  • Ensure that you use GOOD QUALITY co-ax, preferably at least mini-8 or, preferably, the "Fat" low-loss co-ax (RG-213, URM67, Westflex, etc) if you can.
  • Keep the co-ax run as short as is practically possible. Do not have great loops or tangles of spare co-ax laying along the route.
  • Ensure the A99 is well grounded
  • Finally, especially with an A99, I would recommend the Air-Choke[1] (Ugly Balun) ALSO.
    • This not difficult to make: For 11m use, simply wind five turns of Mini-8 co-ax around a 10 or 11cm Plastic former (piece of drainpipe, or even an empty plastic bottle), situated close to the A99 Feed Point.
Very Simple Co-ax Air Choke

Posted Image

[1] When I first used an A99 for SSB, my Voice could be heard on any telephone being used in my house. I tried: installing a low-pass filter; I tried relocated the aerial, tried changing the co-ax, I tried relocating my shack within the house and even tried using a different, brand-new, telephone. Nothing stopped the SSB Voices. I then fitted an Air-Choke and the Voices were gone instantly.
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Attachments: SimpleAirChoke.gif (12.73 KB)
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Gert (19CT006)
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19 Charlie Tango 006

Hi Steve,

Thank you VERY much for your outstanding (and very complete) info about this topic! :thumb:

Of course I'm going to use RG 213. In fact, I have to use that one because that's the only one that will be rigid enough to push it through that rain pipe with all of these tricky corners and angles.... (take a look at my situation in the attached drawing). That is my major concern by the way but I hope I will manage it.

As far as the A99 concerns: if I'm going to mount that one I will certainly take your advices. I told you already that this antenna is a compromise because the aerial that I really want to install is a Gain Master. However the local government has restrictions for antenna length, which is 5 mtrs max. But you know what? Maybe I'm just going to be an anarchist and mount a Sirio after all! In that case I don't have to worry about TVI at all because this aerial is well shielded with it's embedded choke coil. I could combine that with a low pass filter.

I'll keep you posted.

Many thanks again for your reply!

73's, Gert

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Attachments: Aerial_situation.png (18.64 KB)
Edited by Gert (19CT006), Thursday, 18. October 2012, 18:57.
19 CT 006
Operator GERT
Hillegom - The Netherlands

E-mail: [email protected]
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