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 Post subject: Aligning radios with antenna trimmers
PostPosted: Feb Thu 16, 2017 2:10 am 
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Location: Costa Mesa, California
After aligning a few radios with front panel antenna trimmers, it occurs to me that there is a logical way this should be done--and I have yet to see a manual that follows what I believe is the best procedure. It may be that I am wrong about my assumptions and that is why I present it here.

1.) Adjust trimmer capacitor to half capacitance (half meshed)
2.) Loosen antenna trimmer knob and set to the center position
3.) Feed a signal at the center frequency of the band being aligned into the antenna jack
4.) Set the dial to center of band (or find the signal near the center of the band)
5.) Adjust the antenna coil inductance for maximum output

Continue with alignment of the dial for the band as prescribed by the manual adjusting the antenna trimmer as needed for maximum at either end of the band. Do not adjust antenna coil (inductance) further unless the band alignment was so far out that the antenna trimmer alignment procedure should be repeated again at the end, after dial alignment for all the bands.

I am using this method now and it seems to work well and keeps the trimmer centered for each band. When I use a radio after this procedure, I am not trying to remember where the trimmer has to be set for each band--and that is useful as some radios can be pretty dead until the trimmer is properly set. The only drawback I can see is that if the original antenna inductances were so poorly designed that they won't find a peak with the trimmer centered. Anybody have comments or suggestions?

Norm

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 Post subject: Re: Aligning radios with antenna trimmers
PostPosted: Feb Thu 16, 2017 2:17 am 
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Norm,

That is the same basic procedure I follow with the only exception being in rare cases where the manufacturer calls for a specific setting of the trimmer during alignment. One thing I would add is that the best alignment for the input stage is with the actual station antenna attached and align it either using noise or a signal source loosely coupled to the antenna. The load, shunt capacitance in particular, provided by the actual feedline and antenna is often far different than that presented by the signal generator.

I follow the same basic practice when aligning twins like the Drake 4 line or similar where the receiver and transmitter can be slaved together for transceive operation. The most accurate alignment occurs with the actual loads applied by the equipment to the mixers and oscillators.

Rodger WQ9E


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 Post subject: Re: Aligning radios with antenna trimmers
PostPosted: Feb Thu 16, 2017 2:25 am 
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Good insight, Rodger. I considered whether the antenna should be part of the alignment, but for many of my radios, there is no specific antenna in their immediate future.

Norm

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 Post subject: Re: Aligning radios with antenna trimmers
PostPosted: Feb Fri 17, 2017 6:46 pm 
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I'd align the antenna coil to the low end of the band, with the trimmer centered. But, it probably makes little difference so long as you can peak signals at both ends of the dial.

The ant trimmer has much more authority at the high end of the band.

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 Post subject: Re: Aligning radios with antenna trimmers
PostPosted: Feb Fri 17, 2017 8:10 pm 
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Posts: 625
I've done that alignment method with all four of my restored boatanchors.
(The 5th, an RA-17, arrived on my porch 45 minutes ago.)

The part about connecting an antenna is important! The trimmer
position can vary wildly depending on the impedance presented by the
feedline. A resistor of the nominal value may not work unless you're
a ham and have the feedline matched happily to your 50 or 75 ohm
transmitter and never use it at wildly different frequencies.

None of my boatanchor trimmers are out of range for any antenna.
I've checked and the SX-88 is close and on the top two bands
can get unstable with no antenna and some feedline lengths.
Its not unstable with any reasonable antenna, however, at any feedline length.
(Or was it the NC-400?)


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 Post subject: Re: Aligning radios with antenna trimmers
PostPosted: Feb Sat 18, 2017 5:27 pm 
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There are two types of radios with front panel antenna capacitor controls--those with a dedicated main tuning capacitor gang for the antenna coils and those with no main tuning gang. In the first case--an example would be the National NC 183 series--the antenna trimmer is just that, a small air variable capacitance trimmer for the antenna gang that makes adjustments depending on the antenna configuration for optimizing the input. The second case--like the Hammarlund HQ 110 series--does not have a dedicated main tuning gang and the antenna front panel adjustment does the entire job of tuning the input circuits. In that case the "trimmer" is not really functioning to trim the main gang but is acting as a stand-alone independent gang and is a larger air variable capacitor.

In the second example, it is probably more important to center the antenna control before adjusting the antenna input inductance at the frequency in the middle of the band.

The reason trimmer capacitors are adjusted at the high end of the band instead of the low end of the band is simple. At the low end, the main tuning capacitor has much more impact than at the high end where its capacitance is at its lowest point. Adjusting a small value trimmer at the low end isn't going to do much in comparison to the large value of the main tuning capacitor, so finding any kind of correct adjustment is out of the question. At the low end of the band, the balance is really between the inductance and the main cap--which can't be adjusted--so the inductor is adjusted. At the high end of the band, the balance is between the inductance and the main cap / trimmer combination where the trimmer can have influence on where the adjustment ends up.

This points to Pete's comments above as important when adjusting front panel antenna trimmers with dedicated antenna gangs on the main tuning capacitor.

Norm

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 Post subject: Re: Aligning radios with antenna trimmers
PostPosted: Feb Sat 18, 2017 5:30 pm 
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Good point on the radios that don't have a third ganged antenna tuning capacitor.

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 Post subject: Re: Aligning radios with antenna trimmers
PostPosted: Feb Wed 22, 2017 8:23 pm 
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For receivers with a front panel antenna trimmer I will initially adjust the trimmer for the loudest static or station (if I am tuned to one) at wherever I happen to be on the band then as I tune significantly lower or higher I'll re-adjust the trimmer for loudest static and finally when I am tuned into a station I adjust the trimmer again for strongest signal.


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 Post subject: Re: Aligning radios with antenna trimmers
PostPosted: Feb Thu 23, 2017 12:49 am 
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As in a lot of things...it depends! I checked the alignment instructions for my Heath "Mohican" - about the only radio I have that what I would call a "normal" antenna trimmer - a small panel mount variable cap across the RF stage main variable cap. The instructions say to set the panel trimmer to half open before starting. However, also across this trimmer is the normal RF stage trimmer and the main cap and coil. So if the alignment is perfect, then there should be no need to adjust the panel trimmer BUT there is no mention of whether the huge telescopic antenna should be extended or not - I suspect that the capacitance of that will detune the RF stage which can be compensated by the panel trimmer.

I feel an experiment coming on.... :)

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