Mini-Whip antenna for very close by transmitters - Page 1
Forum section: electronics / rf-microwave

author: petert
author_rank: Regular Contributor
timestamp: May 02, 2020, 03:09:11 pm
content: Hello,Assuming you have a Mini-Whip such as this:https://www.ebay.com/itm/2X-Mini-Whip-Mf-Hf-Vhf-Sdr-Antenna-Miniwhip-Shortwave-Active-Antenna-For-Or-E7Z2/303556945380Would you be able to receive signals from close-by transmitters (within a few meters or even less than a meter distance), or will this kind of antenna only work for signals from more distant transmitters and when mounted high-up?The main purpose would be to hook the antenna up to an SDR-receiver to test low-power local transmitters, where both the antenna and transmitter are located on the same bench.

author: fourfathom
author_rank: Super Contributor
timestamp: May 02, 2020, 03:27:27 pm
content: Any antenna, including that amplified mini-whip, will work at short ranges. For your needs, a few inches of wire stuffed into the SDR antenna jack should be sufficient, you don't need the amplifier. Do be careful that you don't overload your SDR with a too-strong signal, so start with a very short wire.

author: cdev
author_rank: Super Contributor
timestamp: May 06, 2020, 07:06:32 pm
content: If I were you I would use a very small loop that protects the input of the SDR from ESDSometimes I use a BNC cable and a BNC to banana plug adaptor with a small piece of wire running from red to black making a loop with around two inches of wire. You may even want to use an attenuator on that. Any kind of transmitter except the weakest oscillator is likely to overload the input of something thats that close.I take great care to protect the input on my mini whip because its VERY sensitive.So reading your title makes my head hurt. You don't need that kind of antenna, you need the smallest "antenna" possible.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: May 10, 2020, 11:35:54 am
content: As I understand your question,you wish to test transmitters on a bench. I don't know what you are testing for exactly, but here are a few things that I use. If you transmit into a dummy load, some of them have a port meant for test equipment, some "Cantennas" like the Heathkit had one. You hook up a scope or freq counter or a Spectrum Analyzer to the port. I have a military power meter that includes a pretty nice 50 ohm load. using the load you can still use the output to monitor the signal. Enough signal "leaks" thru the load to get a nice sample. I have to be careful not to switch to the "thru" mode though, think of all the equipment I would damage.A less involved system involves any decent load and wrapping about 5-10 turns of regular wire around the outside of the coax. Then just connect both ends of the pickup wire to a scope or whatever. Just use a load and sample from the wire wrapped around the coax. Cheap and effective.A more exacting method would be to use big attenuators as the "Load" and sample at the output of the load. Have to be careful to attenuate the signal enough. This is the Professional Approach and lets you analyze the signal and compute the power at the same time.But FIRST:You stated that you wish to use a receiver SO....You may not need any "pickup " at all. Just transmit into a load and see if your receiver picks up the stray signal not absorbed by the load. It probably will. Just use a short wire antenna on the receiver as previously suggested. If you are using a powerful transmitter you may not even need an antenna on the receiver. Then wrap the antenna wire around the coax if you need a better signal. I do this all the time. If you get a bit creative, you can monitor the voice of a transmission as well as the freq, and if you get a bit more involved you can look for harmonics that the transmitter may produce. It is a bit of work to figure this our using only a receiver. That's why spectrum analyzers are used.