Electronics > RF, Microwave, Ham Radio

questions about antennas and sleeve baluns

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Agent Smith:
Hello, I am interested in antenna construction. I have purchased some inexpensive collinear antennas, but I have serious doubts about their validity. Over the past few months, I have tried to delve into this topic and have read hundreds of documents, but there are still some things I don’t fully understand. Can you help me?

For instance at http://www.w8ji.com/sleeve_baluns.htm i've found an explanation of sleeve baluns, but it's not clear if the sleeve connected at the center feed point and running backwards to the line is a radiating element or is radiating when its open end is toward the whip.


I appreciate any help.

Solder_Junkie:
Rather than relying on guesswork and speculation, a common issue for those discussing common mode currents on coax cables, why not measure it?

A common mode current meter that works up to 2m and is within 20% from 1.8 to 70 MHz, is really easy to make. This site shows you details, including calibrating the meter:
http://www.ifwtech.co.uk/g3sek/clamp-on/clamp-on.htm

Ferrite sleeve chokes are effective, those in the UK can buy useful sleeves from CPC, item 559-568, cost is £1.61 + vat for a pack of 5. These slide over RG213 size cable and provide a measured 19 dB of common mode current attenuation on 2m (using 5 ferrites).

Antennas-Amplifiers use a small coil of coax on their 2m Yagi antennas, Tonna used an aluminium tube grounded to the boom at the end away from the driven element, and Innov use ferrite sleeves. On none of these antennas can I measure any common mode current. Incidentally, the sleeve on the Tonna Yagi is shorter than a 1/4 wave in free space, as it will be effected by the coax through the sleeve.

SJ

Agent Smith:
(sorry for my language, i don't speak english very often)
First, thank you for your reply.
I love multimeters and i'd like to have another one more, but i'm sure there is a common mode current because the measure with nanovna is very unstable and looks like a fine V shape when i touch the line with my hand. I just received some toroids from ebay, anyway my answer here is just to understand, to know what to do in the future, and so.

My only operating frequency is 869.5 MHz, i have three enclosures and i should fit them with different elements combinations to have a choice among three different gains or radiating lobes. If the sleeve balun is a radiating element i won't hide it under the enclosure aluminum base, if not i can save 8 valuable centimeters. Unfortunately i have only 0,15W of power, everything should be optimized.

Solder_Junkie:
150mW at 860 MHz is not going to show on a simple common mode current meter.

Your reference to W8JI appeared to refer to amateur radio antennas and power levels (up to 1500 Watts, depending on frequency and country/licence). Clearly this is not the case.

At 150mW, I wouldn’t bother about common mode currents.

SJ

pienari:
Is it LORA or some other system?

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