# Partially Collapse Telescoping Antenna to Change Effective Length

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 for NOAA APT reception?

[I also posted this question on Amateur Radio Stack Exchange but it's still in beta so I want to make sure I get some answers :)]

300 MHz has a wavelength of precisely 1 metre and, a quarter wave dipole (a telescopic antenna for example), would need to be 25 cm long at 300 MHz. At 170 MHz it would need to be $$\\frac{300}{170}\$$ times longer at 44 cm.

Is there something special about telescoping antennas such that they only work when fully extended, or can I partially retract

You can retract to the length you want but I wouldn't expect it to be that good if you had to retract it to just a few cm because it's effective diameter would be significant but I don't see it would be a problem for you usage. Try it and see.

Many sites with instructions for a V-Dipole antenna for this purpose indicate that a 53cm half-wave antenna works well.

Hmm! They probably work fine - it's not so critical for a receiver as it is for a transmitter.

• So there's nothing special about telescoping antennas such that they wouldn't work correctly when partially retracted? Dec 24, 2018 at 17:51
• I believe I answered that question. Dec 24, 2018 at 17:52
• There is no risk of VSWR damage to the receiver, by experimenting with the antenna length; ESD events are the only risk, so touch metal ground on the SDR before and as you make length changes. Dec 24, 2018 at 18:01
• Your numbers here are not actually correct, because you are mistakenly using the speed of light in vacum. The velocity factor applicable in an antenna actually makes it somewhat shorter. Dec 24, 2018 at 19:59
• Yes I agree but for the receiver it won’t make much of a change to SNR. Dec 24, 2018 at 21:46

Getting the length right for a proper impedance match is just one step in achieving a good signal.
The coax "hot" wire goes to the whip; a ground plane goes to the coax shield - this is the other part of a monopole antenna.

A dipole is self-contained and doesn't need a ground.

You might also look at antenna pattern - weather satellite signals at 137 Mhz can appear almost anywhere in the sky on various passes (their orbits are low and pass by fast). Many users strive for fade-free reception during the five-to-fifteen minutes that they're available.
A monopole standing straight up has a null directly above - not desirable. A horizontal dipole has a signal maximum directly above - much preferred. But the dipole has a null off its ends, toward the horizon. A bent dipole might help.

Both these antennas are polarized in one direction. Many satellites transmit circularly-polarized signals in an effort to reduce fading that puts noise streaks in your reconstructed image. So both types of antennas are susceptible to fading. Crossed dipoles (turnstiles) can help reduce this problem, but are more complex to construct properly.