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I'm looking at the CZE-05B FM transmitter and in the manual it states the output load is 50 ohm and I need an 50 ohm antenna.

So my questions are is this antenna 50 ohms? https://www.ebay.com.au/itm/2121-Telescopic-Extendable-Antenna-BNC-Connector-for-Portable-Radio-Scanner/392126393923

I also plan to follow the following guide https://vk1nam.wordpress.com/2m-dipole-for-sota/ to make an an dipole antenna.

I have read I might need a Balun can anyone offer some advice please

Thanks

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  • \$\begingroup\$ I don't think I would risk connecting a transmitter to that telescoping antenna. It may be 50 Ohms, but I doubt it is 50 Ohms over the whole band and at all levels of extension/retraction. It is possible to damage a transmitter by connecting it to an antenna not properly tuned to the transmit frequency. In any event, if the seller can't provide you with basic technical information such as VSWR and so-on, then you should look elsewhere. \$\endgroup\$ – mkeith Nov 19 '18 at 6:00
  • \$\begingroup\$ I'm voting to close this question as off-topic because we're not the ones you should be asking about product specifics, but the seller. \$\endgroup\$ – Marcus Müller Nov 19 '18 at 9:44
  • \$\begingroup\$ I mainly want to understand how to determine the ohms of an antenna and if that one and the one i plan to make will be close enough to 50ohm to work \$\endgroup\$ – Uskompuf Nov 19 '18 at 9:48
  • \$\begingroup\$ Basically, what you need is a standing wave ratio (SWR) meter for VHF band. Ham radio operators use SWR meters to determine when their antenna is well matched to their transmitter. For sure the antenna is NOMINALLY 50 Ohms, but it will depend on which frequency, and how far the antenna is extended, and what metal surfaces are nearby. Basically it is a really crappy transmit antenna. You should make a better one, and you should also buy an SWR meter and read up on these topics. \$\endgroup\$ – mkeith Nov 20 '18 at 5:35
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A whip antenna, in theory, is resistive at 1/4 wavelength (perhaps slightly shorter in practice) when it is mounted on a ground plane. At 100 MHz the wavelength is about 3 m so a 1/4 wave whip is going to be around 75 cm. So this antenna at 45 cm is too short. You will also need a ground plane or radials to approximate a ground plane. At resonance the theoretical resistance is about 37 ohms, which is usually considered close enough to 50 ohms. As @mkeith says, the match will worsen away from resonance, being capacitive below and inductive above. Probably need to retune for different frequencies. You don't need a balun for a whip. Just connect the outer of the coax to the ground plane at the base of the antenna. On the other hand, a dipole is balanced so you need a balun for best performance. A half wave dipole is about 75 ohms at resonance, also probably close enough to 50 ohms in practice. A balun, as the name suggests, is usually a balanced to unbalanced transformer on a small ferrite core. You can also wind a few turns of coax onto a ring core for a reasonably good balun. There are plenty of examples out there.

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  • \$\begingroup\$ we just need an antenna to test while we make our dipole any recommendations for a very cheap 50ohm antenna for testing? Thanks \$\endgroup\$ – Uskompuf Nov 19 '18 at 9:46
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    \$\begingroup\$ Depends what testing you want to do. You could use a 47 ohm non-wirewound resistor as a dummy load. Short leads. You could use a resistive attenuator of say 7 dB or more and plug a random piece of wire into the output. The impedance of the random piece of wire would then not matter to transmitter as it would never see more than 14 dB return loss. \$\endgroup\$ – Steve Hubbard Nov 19 '18 at 23:23

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