guidance for asking antenna related questions improves chances for an answer -proposal for a sticky post

Discuss commercial and home made antennas.
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guidance for asking antenna related questions improves chances for an answer -proposal for a sticky post

Post by snn47 » Sat Aug 24, 2019 9:41 am

Rev.1 2019Aug.24
I propose to make a sticky post that provides guidance about how to formulate antenna related questions.
This makes it easier to no what information is needed, without having to consult a crystal ball if you are lucky enough to have a funtional one. This makes it is easier to answer and keep it simple, while it improves the chance that the author will get the answer he is looking for. For example "what is the best antenna for me" cannot be answered without by anyone.

Here is my proposal for your comments/improvements, of what I consider basic information concerning antenna questions:

- is the antenna needed for indoor or outdoor use?
explanation: Only antennas very high above ground are not subjected to reflective surfaces and objects which will deform antenna elevation and azimuth pattern change bandwidth and antenna gain. Indoor only very small antennas have some chance of providing some performance, while large ones that fill the room are more likely to not perform at all. All depends also on the building material e.g. wood or stone and how many wires, tubes or other material will interfere with an antenna.

- What polarization of the signals, vertical, horizontal, circular or elliptical?
explanation: Antennas have different kind of polarization, compared to polarized sunglasses which will suppress most reflections. Turn polarized the sunglasses 90° and you see only reflections. Such cross polarization losses e.g. horizontally vs. vertically polarized, are theoretically 20 dB and from what we measured at VHF at least >8 dB if additional bending/scattering of signals improves the situation.

- Should the antenna have a circular/omnidirectional or directional coverage?
explanation: Any increase in antenna gain will result in a reduction of the antenna beam width in vertical and/or horizontal direction.

- Are the signal sources on ground, airborne or satellite based?
explanation: For ground sources increase in gain at lower elevation angles improves reception while this will in consequence reduce reception for most airborne or satellite based signal sources unless just above the horizon.

- What is the frequency range the antenna should perform well or is the desire to provide at least some reception over a wider range?
explanation: Bandwidth and gain limit, mean either high gain you can have a high gain in a defined pattern meaning reduced azimuth or elevation width or you get one to two octaves of bandwidth but 0 dBi or lower gain. Exception to those guidance exist in form of active antennas, but their large bandwidth will create other problems like overloading in the receiver without adding Low-Pass (LP), High Pass (HP), Band Pass (BP) or Band-Reject- (BR) -Filter.

- size limitations, limitations of the location or antenna mounting?
explanation: Antenna gain and vertical and horizontally radiation pattern, are unless they have been measured for a defined location, when placed in freespace without any conductive/reflective objects. When place on/above ground/building reflections will overlap with the direct path freespace antenna pattern. The resulting antenna elevation will be totally different and vary with many factors like ground conductivity, terrain slope. The same applies to the azimuth pattern which why only obstacle free placement will provide somewhat omnidirectional pattern.
Then there are mechanical limitations like weight for the mechanical antenna mounting and wind resistance that the antenna mounting should be able to withstand.

- is Antenna needed for transmit or receive only?
explanation: Since we have an increasing of transmit capable SDR it is not obvious that an author wants receive only.

Unless the receiver (e.g. RTL-SDR) is directly connected at the antenna feed point there is also the transmission line to consider the loss of the transmisson line length vs. loss per meterbetween antenna and receiver:

(e.g. cable type type and impedance intended for use

consider the loss of the transmisson line length and receiver
For coax cable as transmission line the the loss [dB] in dB per meter between antenna is multiplied by the cable length, plus adda few tenth of a dB for each connector used.
Whatever real gain an antenna might have, using high loss transmission lines (e.g. thin coax like RG174, high loss versions of coax or cheap coax knockoffs, with just a few strands of wire whithout providing any real shielding)

do antenna, connector and transmission line have the same impedance
Antenna, transmission line (e.g. coax) and receiver should have identical impedance.

PS.: Please use the search funtion first and look through available posts for answers.
Many antenna related questions repeat itself,e.g. QFH antenna related ones, and so do the answers, information and links provided.
If something is missing, why not ask the questions in such an older post so that information is just in one post, and not scattered here and there.
Last edited by snn47 on Thu Sep 05, 2019 4:56 pm, edited 3 times in total.

Posts: 43
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Re: guidance for asking antenna questions improves chances for an answer -proposal for a sticky post

Post by HighSNR » Sat Aug 24, 2019 9:55 am

All seem reasonable to me. I'm sure there lots of other info people could volunteer too.

Just telling us the frequency ( in Hz) is a big help. Don't make us research that esoteric signal that may even be specific to your country.

Cable lengths and types used is also worth knowing. What type(RG number) What characteristic impedance.

Any info on the required link budget really helps. What SNR is required. Bandwidth, Rx sensitivity.

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Joined: Sat Jan 14, 2017 11:07 pm

Re: guidance for asking antenna questions improves chances for an answer -proposal for a sticky post

Post by hotpaw2 » Sat Aug 24, 2019 6:10 pm

Other important antenna issues might involve the possible need to get the antenna far away from any local RFI/EMI noise sources (PC, LED lamps, power supplies, etc.), and possibly the use of choke baluns to prevent coax/feedline problems (picking up local EMI/RFI, changing the antenna pattern, etc.)

Posts: 166
Joined: Tue Dec 27, 2016 11:00 pm

Re: guidance for asking antenna related questions improves chances for an answer -proposal for a sticky post

Post by snn47 » Sat Aug 24, 2019 6:53 pm

@ High SNR
Added your suggestions

@hotpaw and others
Are you proposing to add this information here or as separate post, e.g. Must read on antenna and transmission lines?

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