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Adding an antenna trimmer to a receiver




 
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Scott SWL
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« on: March 30, 2021, 02:13:39 AM »

I have several shortwave radios, one of them has an antenna trimmer control(Halicrafters SX-99) which seems to work nicely with a 25ft piece of wire, and the other antenna terminal tied to ground.
I looked at the schematics, and it is just a air variable cap to ground at the 1st RF amp input(but after the tuning stage).
I would like to add this to the other radios I have.
Is there a way to make this external to the radio,across the antenna terminals?

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Carl WA1KPD
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« Reply #1 on: March 30, 2021, 09:15:58 AM »

They tend to be parallel with the RF amp tuning cap.
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Carl

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n4joy
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« Reply #2 on: March 30, 2021, 10:42:59 AM »

Have you considered constructing a simple Pi section antenna tuner?  That's what I did for my SX-85. 
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« Reply #3 on: March 31, 2021, 06:50:25 PM »

Your antenna has an inductance of approx. 13.2 uH, so with a variable capacitance of 10 to 650 pF cap in series with the antenna you could tune bands 160m to 20 meters.

The tuning capacitance would have to be isolated above ground with a big knob on the shaft.

Fseries = 1/(6.28 X SQRT(Lant X C))

Phil - AC0OB

* SWL Antenna Tuner PDF.pdf (188.92 KB - downloaded 32 times.)
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Scott SWL
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« Reply #4 on: March 31, 2021, 08:50:26 PM »

Thank you!
I'll try it
Horrible interference around here, I'm trying to build something for inside the apt so I can listen.
From broadcast to 30mhz
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« Reply #5 on: April 01, 2021, 01:36:21 PM »

Thank you!
I'll try it
Horrible interference around here, I'm trying to build something for inside the apt so I can listen.
From broadcast to 30mhz

One could use a ceramic switch (also isolated above ground) that adds-in capacitance for the BC band, similar to the one below:

https://www.tubesandmore.com/products/capacitor-3x365pf-variable-3-section
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Scott SWL
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« Reply #6 on: April 01, 2021, 03:32:13 PM »

This is basically what I'm looking for.
Is there a chart, of wire length, and capacitance to resonant frequency someplace.

I have a couple of variable air caps already 100 and 140pf but I may have to get the one from AES if they aren't enough.
I'm Thinking something like this, but with switched coil lengths


* antena.jpg (49.61 KB, 480x640 - viewed 13 times.)
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« Reply #7 on: April 01, 2021, 03:46:36 PM »

See section 2.2.2 of the first reference:


https://www.physics.princeton.edu/~mcdonald/examples/cap_antenna.pdf

https://www.antenna-theory.com/antennas/smallLoop.php

https://www.ece.mcmaster.ca/faculty/nikolova/antenna_dload/current_lectures/L12_Loop.pdf
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Scott SWL
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« Reply #8 on: April 01, 2021, 04:30:51 PM »

Thanks!, I've got some reading to do. I'm off the next 2 nights, I'll do it then.
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WA4WAX
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« Reply #9 on: April 02, 2021, 11:43:49 AM »

Sharp Phil included a 1Meg static drain.  Many  would omit.

Natalia (McMaster) is  top shelf.  She knows about electric sound  waves.  :-)
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Scott SWL
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« Reply #10 on: April 02, 2021, 12:25:31 PM »

Unfortunately, this math is so far over my head. Does it have to be so complex just for a receive antenna? Much of what is in the math has to do with radiation of power and losses, and not resonance.
I will just look for plans for an antenna.
Also these calculations seem to be for a single loop, and not a spiral coil , and I'm not sure if it applies.
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WA4WAX
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« Reply #11 on: April 03, 2021, 11:54:43 AM »

Here is more.

http://haralick.org/conferences/maxwell_slides_ESTC.pdf
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« Reply #12 on: April 03, 2021, 04:06:29 PM »


I will just look for plans for an antenna.
Also these calculations seem to be for a single loop, and not a spiral coil , and I'm not sure if it applies.

What you showed in your picture in post #6 was a square "loop," not a spiral coil.

Section 2.2.2 of the first link has equations one can do on a calculator.

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Scott SWL
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« Reply #13 on: April 03, 2021, 04:17:17 PM »

There are more than 20 equations in that section, and none that I understand will give me the length of wire and capacitor value for tuning a frequency range.
Some designs also have an inductor as well.
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WBear2GCR
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Brrrr- it's cold in the shack! Fire up the BIG RIG


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« Reply #14 on: April 03, 2021, 05:33:09 PM »

Scott,

25 ft wire?

A few basic questions.
Is this the best, aka longest, piece of wire possible at your QTH??
Is it outside?

Regardless, in a restricted location, one of the best ways to go is with
a Magnetic Loop antenna. This is not quite the same as any loop
antenna.

There are a LOT of sites with designs to copy, and quite a few (maybe over priced)
sellers of pre-made loops.

I would suggest that you build a loop from some plumbing tubing - the largest diameter
you can manage, even if it is inside. Or some RG-8 size coax, or the like...

You can use very low voltage parts, as you are not transmitting!
So, small trimmer caps will work fine. Even home made fixed or variable caps can have
"index card" thin spacing, as you are dealing with no voltage!

Most of the designs are for standard or QRO transmit power handling, and many for
QRP transmitting. You can use parts that are even smaller than those, if you wish.
Receiver variable caps and trimmer caps are fine.

Mag loops have the nice ability to get rid of a lot of noise and concentrate on one
frequency...

Otherwise, you can get some benefit from an outboard "preselector". The MFJ people and
others make and have made both "active" (preamplified) and "passive" units. As previously
mentioned they're similar, usually, to a "PI Network"... So, you can build your own, and
put it in a box, or screwed to a wood board. So, some variable receiver caps, a coil and some wire and connectors.

                    _-_-bear
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« Reply #15 on: April 03, 2021, 05:35:27 PM »

---please delete this inadvertent post---   (don't see a way to do that!)
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« Reply #16 on: April 03, 2021, 07:08:04 PM »

There are more than 20 equations in that section, and none that I understand will give me the length of wire and capacitor value for tuning a frequency range.
Some designs also have an inductor as well.

Then this amplified (active) loop may be just what you need and has already been designed for you so you don't have to deal with simple algebraic equations:

https://mfjenterprises.com/products/mfj-1886?_pos=7&_sid=7d515a2a0&_ss=r#product-manuals

https://cdn.shopify.com/s/files/1/0289/7782/3843/files/MFJ-1886.pdf?v=1586534167

See page 4 of 8 on how to connect it to your receiver.


Phil
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Scott SWL
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« Reply #17 on: April 03, 2021, 08:11:33 PM »

Scott,

25 ft wire?

A few basic questions.
Is this the best, aka longest, piece of wire possible at your QTH??
Is it outside?

Regardless, in a restricted location, one of the best ways to go is with
a Magnetic Loop antenna. This is not quite the same as any loop
antenna.

There are a LOT of sites with designs to copy, and quite a few (maybe over priced)
sellers of pre-made loops.

I would suggest that you build a loop from some plumbing tubing - the largest diameter
you can manage, even if it is inside. Or some RG-8 size coax, or the like...

You can use very low voltage parts, as you are not transmitting!
So, small trimmer caps will work fine. Even home made fixed or variable caps can have
"index card" thin spacing, as you are dealing with no voltage!

Most of the designs are for standard or QRO transmit power handling, and many for
QRP transmitting. You can use parts that are even smaller than those, if you wish.
Receiver variable caps and trimmer caps are fine.

Mag loops have the nice ability to get rid of a lot of noise and concentrate on one
frequency...

Otherwise, you can get some benefit from an outboard "preselector". The MFJ people and
others make and have made both "active" (preamplified) and "passive" units. As previously
mentioned they're similar, usually, to a "PI Network"... So, you can build your own, and
put it in a box, or screwed to a wood board. So, some variable receiver caps, a coil and some wire and connectors.

                    _-_-bear
Thanks Bear!
I live in an apartment, so no outside antenna is possible unless I can hide it under the vinyl siding. I magnetic loop is what I'm looking for I think, like the pic in post 6 where the coil begins in the center, and spirals out Square is the easiest for me to make, by selecting points along the spiral, I should be able to switch active frequencies I have seen some plans that use a Pi network as part of the tuning system.
I, like most people in this hobby like to build my own stuff. I am just restarting with shortwave after 35 years. I used to have a Collins 75a 1? I think, it was a nice radio, and there was little interference when I lived at my parent's house.
My other hobby is tube audio.
I have never transmitted except for CB when that was a thing.
I also had the matching KWS1 transmitter, but I only turned it on, without keying the mike.

Scott
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Patrick J. / KD5OEI
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« Reply #18 on: April 04, 2021, 01:13:12 AM »

One thing not mentioned so far is the 'multiband tuner'. This simple circuit covers nearly the whole HF band in a 180 degree turn of the capacitor.

You would want a reduction drive on the tuning cap for that.

It offers fairly sharp tuning, high impedance to wanted signals and very low impedance to whatever it's not tuned to, being a series or parallel resonant circuit.

These are shown in ham magazine articles, usually covering 10 through 80 meters. The range can be extended using a switch to add inductance to the coils or the variable capacitor. Commercial examples are the national MB-40 and MB-150, but the design is pretty simple.

There is an article on this sort of thing posted here on AMfone in this topic:
http://amfone.net/Amforum/index.php?topic=30008.msg233386#msg233386

Though the article shows a push pull or balanced circuit, one can dispense with the lower half for single ended circuits. The values remain the same, per side.

Also see "Vernon Chambers" who wrote articles on that and popularized it. And very little math.

also:
http://www.arrl.org/files/file/Technology/tis/info/pdf/5401011.pdf
http://jlandrigan.com/files/MB40%206146%20in%20Push%20Pull.pdf

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WBear2GCR
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« Reply #19 on: April 04, 2021, 12:40:12 PM »


Thanks Bear!
I live in an apartment, so no outside antenna is possible unless I can hide it under the vinyl siding. I magnetic loop is what I'm looking for I think, like the pic in post 6 where the coil begins in the center, and spirals out Square is the easiest for me to make, by selecting points along the spiral, I should be able to switch active frequencies I have seen some plans that use a Pi network as part of the tuning system.
I, like most people in this hobby like to build my own stuff. I am just restarting with shortwave after 35 years. I used to have a Collins 75a 1? I think, it was a nice radio, and there was little interference when I lived at my parent's house.
My other hobby is tube audio.
I have never transmitted except for CB when that was a thing.
I also had the matching KWS1 transmitter, but I only turned it on, without keying the mike.

Scott


No - antennas don't quite work the way you have expressed it.

I strongly suggest that you investigate the MAGNETIC Loop Antenna.
Almost certainly it will give you the best performance you can get - even if all you can do
is to build it to hang in your window!

It's a special class of antenna.
AND, it acts as its OWN PRESELECTOR!!

Now, thin wire is almost invisible to the eye.

I can't see your QTH, but I would expect that there may well be a way to go with
a rather longer than expected wire of extremely thin gauge.

If you PM me, we can discuss privately?

I also do audio and tube audio - so maybe there is something to talk about there too.

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Scott SWL
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« Reply #20 on: April 04, 2021, 03:47:27 PM »

I'll get in touch, I have to work the next 4 nights, then I have some time off.
I admit I know little about RF and antennas I am feeling my way in the dark right now.
I've recapped the SP-400, and aligned it, and it seems to work properly at another location The Hallicrafter SX-99 still could be better involving the crystal filter. (whines and howls a bit), but I'm getting awesome sensitivity.
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