Scanner Antenna Tuner DIY Build Question...

[Mikejo]

I've been looking into some 2 meter HAM Antenna tuners to tune/match my scanner antenna. Some DIY designs incorporate two connectors, a multi selectable switch, a tapped coil inductor, and an air variable capacitor.

I'm thinking of building one, but air variable capacitor's are hard to come by these day's (Not a lot of old radios in the trash around where I live), so my question is this: "Is an 'air variable capacitor' the only type of variable capacitor that can be used, or can other type's be used as well?

Keep in mind, this tuner would only be used for receive only!

 

[RFI-EMI-GUY]

There are compression variable caps that can be fitted with a knob by replacing the screw with a longer screw and standoff. . But, you can make a variable capacitor with the right materials. Also, you can buy boat anchor radio equipment and strip out the variable caps.

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[majoco]

Quote from the Eham forum:

The reason nobody uses 2m antenna tuners is because nobody needs one. When you're dealing with antennas only 39" long, anybody can tune an antenna by trimming its dimensions or adjusting its gamma tap point (or delta, or beta, or whatever it has) so it becomes a perfect match on its own without the need for an "antenna tuner."

A tuner for VHF or UHF antennas is silly and also very uncommon. Tune the antenna itself by adjusting it, not by using a "tuner."

An antenna tuner will almost defeat the object - to scan a broad selection of frequencies - as it will peak up only a narrow band - you will spend all your time retuning the tuner!. If you want to listen to only a small frequency band, then make/buy an antenna for that band - if you want to scan a wide range, then get a wideband antenna - a discone perhaps

 

[Mikejo]

Actually, that is the whole point of my making the tuner! I want to fine-tune every frequency in the spectrum I would be scanning. I especially don't want To build an antenna for each and every frequency I want to park on!

 

[Mikejo]

I actually know the frequency or frequencies I want to park on and monitor!

 

[prcguy]

In my experience with a Sti-Co brand VHF antenna tuner, it made little difference if any in reception and its only useful to help match a transmitter to an antenna that has a questionable match.

The Sti-Co and similar VHF tuners are a Pi type with two very small variable capacitors probably around 50pf each and a very small one turn coil. At VHF these parts are very tiny compared to an HF tuner. Unless you have a lot of experience building components at VHF frequencies, you can easily introduce more loss in the path that you might tune out with a well designed tuner.

What kind of antenna are you using that needs a tuner?
prcguy

Actually, that is the whole point of my making the tuner! I want to fine-tune every frequency in the spectrum I would be scanning. I especially don't want To build an antenna for each and every frequency I want to park on!

 

[Mikejo]

I'm using a J-pole and I've actually seen these tuners in action and that's good enough for me!

What I really really need to know is, will another type of variable capacitor work? Or is the only capacitor that will work in this instance is in air variable capacitor? That's the only information I'm looking for.

 

[RFI-EMI-GUY]

Actually, that is the whole point of my making the tuner! I want to fine-tune every frequency in the spectrum I would be scanning. I especially don't want To build an antenna for each and every frequency I want to park on!

OK, what you want to build is a preselector? You are no so much concerned with the antenna match as you are separating a desired signal from paging or NOAA weather interference?

However, scanning and an external preselector are mutually exclusive. Some high end receivers like Icom R9000 have tracking preselector that tunes as it scans.

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[W3DMV]

. You can use some silver mica capicators with a rotary switch to
select the proper appropriate value. since you loose your fine tuning
you would need a variable inductor which are even more difficult.
I have built several tuners for 146 mhz which work reasonably well
but they are very narrow devices, perhaps 2-3 mhz.
your really in a dead alley....

 

[Mikejo]

Ok, apparently I was not as clear in saying what I am going to use this device for. I have made a J-Pole antenna cut for the middle of the AAR (Railroad) Frequency spectrum. When one of those channels is coming through weak, I would like to use the tuner to make the antenna more closely resonant at that frequency to pull it in.

This device will only be used for Receive ONLY.

I know all the current tuners on the market are basically made for 2 meter HAM operation, mainly used to get a 1:1 match for transmission purposes.

Now, I'm sure to hear a lot of bloviating on the merits of whether or not both transmission and receiving are effected and it doesn't matter for receiving at all Blah, Blah, Blah....

The only question I have is if the "Variable Air Capacitor" is the only type of variable capacitor that will work for this basic circuit (see my original post) or not!

If there is another type that will work (Remember for RECIEVE ONLY), I would like to know. Otherwise, all other technical bloviating about theory, fact and whatever will go in one ear and out the other.

I do appreciate the time taken to answer the question, I just do not want to get tied up in an extended and complicated lengthy wad of confusing information.

 

[Mikejo]

...you would need a variable inductor which are even more difficult.....

I will just make an inductor and tap it a 4 or 5 or 10 points(However many switch positions the switch I acquire will have).

I have seen some designs with nice variable inductors, but that would add a level of complication I do not want to deal with. I will be making the tuner with scraps , I really do not want to spend much money on this, it's just being made for fun and learning.

 

[prcguy]

Since you are not transmitting you can use most any variable capacitor that has an appropriate range of capacitance. An air variable or a mica compression or a piston type will all work. You might look up a schematic diagram for a VHF antenna tuner like a Sti-Co or Leader and maybe Yaesu or Kenwood might have made a 2m antenna tuner and see what values they use.

The parts will be fairly small and you have to pay attention to things like lead lengths and grounding at VHF.
prcguy

I'm using a J-pole and I've actually seen these tuners in action and that's good enough for me!

What I really really need to know is, will another type of variable capacitor work? Or is the only capacitor that will work in this instance is in air variable capacitor? That's the only information I'm looking for.

 

[Mikejo]

Thanks prcguy

That's what I needed to know.