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I recently purchased a cheap RTL-SDR kit to experiment with SDRs and different frequency bands. This kit came with 23cm and 100cm telescoping antennas. I now wish to use the 100cm antenna to pick up NOAA APT signals at 137 MHz. Many sites with instructions for a V-Dipole antenna for this purpose indicate that a 53cm half-wave antenna works well.

Is there something special about telescoping antennas such that they only work when fully extended, or can I partially retract the 100cm antennas to 53cm to get the proper length?

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  • $\begingroup$ Take note that for a dipole, the length would be ~54 cm per side.(per leg). $\endgroup$ – Glenn W9IQ Dec 24 '18 at 19:02
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Yes, you can partially collapse a telescoping antenna. There are no special considerations versus other types of antenna elements / monopoles.

There is a slight effect from the thickness of the antenna — thicker conductors, such as the telescoping elements closer to the base, exhibit more bandwidth (less selectivity). This is usually not very significant at all for receiving purposes, but if you're trying to pick out a single signal then you can theoretically get a small reduction of out-of-band interference by extending the thinner sections of the antenna rather than the thicker ones, provided that you also get the length exactly right (adjusting to maximize the power of the received signal).

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Telescoping antennas work at any length you set them. They are always connected internally.

Speaking from experience, being able to move the antenna around to find the best signal makes a much bigger difference than fine tuning its length.

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53cm is ~1/4 wavelength on 137 MHz. I can't tell if your antenna is a single vertical monopole element or a rabbit ears type dipole. Assuming it is a single vertical element, if you provide a ground plane and set the length to 53 cm, you will have a resonant ground plane vertical on ~137 MHz. The ground plane could be a pizza pan or metal filing cabinet, etc. and coupling to it can be either coax braid direct wiring or capacitive coupling through magnets (if any).

If it is a rabbit ears type dipole, each element needs to be set to 53 cm and you can skip the ground plane.

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Radio frequency currents flow largely along the surface of a conductor due to skin effect. As a result, the center portion of a conductor typically plays no role in the conduction of RF current. The conductor can be hollow or filled with any other material and it will have minimal effect, provided the surface conductor is sufficiently thick.

Your telescoping antenna also exhibits skin effect. As a result, the part that is telescoped inside another section plays no practical role in conducting RF current to your SDR antenna input - it is as if it is not even there. Only the visible, outside surface of the telescoping element functions as the antenna.

When configuring a dipole antenna, the overall length in meters is given as:

$$l=\frac{142.5}{f} \tag 1$$

where $f$ is the frequency in MHz.

Equivalently, the length of a dipole in feet is given as:

$$l=\frac{468}{f} \tag 2$$

The length of each leg is one half of $l$. The length is typically not extremely critical for receive applications.

The NOAA APT transmissions are right hand circularly polarized satellite signals (RHCP). The dipole antenna is linearly polarized. As a result you will experience about 3 dB of signal loss. This is typically an acceptable loss for this application. Keep your dipole antenna horizontally orientated and at least 1 meter away from other objects including the earth. Turn it about the vertical axis that is perpendicular to the telescoping elements for the best signal strength.

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