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antenna related information and guidance for asking questions to improve chances for an answer

Posted: Sat Aug 24, 2019 9:41 am
by snn47
Rev.2 2019.Oct.09 Rev.1 2019.Aug.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? 2019.Oct.09
explanation:
Outdoor: Only antennas very high above ground are not subjected to the impact of reflective surfaces and objects which will
- deform antenna elevation and azimuth patterns
- change bandwidth and
- antenna gain.
=> For any outdoor antennas a good lightning protection is adviseable

Indoor the impact on an indoor antenna originates from floor, windows and walls, wires and tubing and the effect the material used has on the antenna pattern. The higher an appartment/room is the better, with a rooftop appartment giving the best chance for reception.

Then there is the building entry loss to consider to, which again depends to a large degree, on the attenuation of the waves when entering the building. How good indoor reception can be depends on the individual situation, meaning e.g. the building loss entry (steel reinforced concrete, bricks or just plywood?), how many wires, tubes or other attenuating material will interfere with an antenna.

Then also windows differ in attenuation, the highest attenuation often have those mirroring windows with metal vapor.
I found that small antennas in relation to the height of the room provided better performance, while large ones that fill the room were not performing except for tuned magnetic loops.

- 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 load that the antenna and 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

- transmisson line loss (cable and connectors)vs. antenna gain 2019.Oct.09
=> guidance for choosing coax cable can be found here viewtopic.php?f=6&t=4861&p=12432#p12432
explanation: All transmission lines (coax-cable or parallel-wire) have an attenuation which increases with frequency.
The overall attenuation of a transmission line and connectors can be calculated by multiplying the attenuation for the operation frequency of the coax cable type used specified by the manufacturer in dB per 100 meter [dB/100 m] or dB per 100 ft (dB/100 ft) by the length of the cable used.

Each connector used between antenna and receiver will add a few tenth of a dB of attenuation, the same applies to adapter e.g. those used to convert from one connector type to another. The actual attenuation of connectors varies with the connector type (e.g. simple coax connector, threaded types like N, TNC, SMA, or Bayonet types like C, BNC, ) or for how many connections they are rated, some versions for as low as only 5 connections.

Any gain obtained by an antenna is easily offset to a large degree by the attenuation of a few meter of high loss transmission lines on higher frequencies (e.g. thin coax like RG-174)

For example you connect a monopole optimized for 1000 MHz with about 5.25 dBi via 5 m of coaxcable to a receiver

If you use RG-174 from Fairview which specifies the max. frequency as 1000 MHz and an attenuation of 1.0499 dB/m (https://www.fairviewmicrowave.com/image ... /RG174.pdf ), the cable loss for 5 m of their RG-174 would be 5.25 dB and compensate all of the antenna gain.

If you use a more rigid, but low loss ¼ inch Cellflex coax cable instead e.g. LCF14-50JFN from RFS which specifies the attenuation of 0.139 dB/m the attenuation is reduced to just 0,69 dB for 5 m LCF14-50JFN, leaving you with an antenna gain of 4.5 dBi.

Therefore it is often more sensible to use coax cable that provides low loss at the frequency you want to receive at, than trying to increase antenna gain to compensate cable loss.

Please note: unfortunately not all coax cable of the same type (e.g. RG-58) will have similar loss. There are also cheap knock off copies, with just a few strands of wire as shield without providing any real shielding, and do not reach the low attenuation by brand name manufacturers.

- 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.

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

Posted: Sat Aug 24, 2019 9:55 am
by HighSNR
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.

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

Posted: Sat Aug 24, 2019 6:10 pm
by hotpaw2
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.)

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

Posted: Sat Aug 24, 2019 6:53 pm
by snn47
@ 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?

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

Posted: Wed Oct 02, 2019 5:24 pm
by nizz -freq scan noob
snn47 wrote:
Sat Aug 24, 2019 9:41 am
Rev.1 2019Aug.24

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)
I'm confused with this sentence :?: :lol:
"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) "

...using high loss transmission lines... is doing what? missing end of this. :)

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

Posted: Wed Oct 09, 2019 7:57 pm
by snn47
@nizz -freq scan noob
I tried to rephrase and provid an example. Does this explain it better?

- transmisson line loss (cable and connectors)vs. antenna gain
guidance for choosing coax cable can be found here viewtopic.php?f=6&t=4861&p=12432#p12432
All transmission lines have an attenuation which increases with frequency.
The overall attenuation of a transmission line and connectors can be calculated by multiplying the attenuation for the operation frequency of the coax cable type used specified by the manufacturer in dB per 100 meter [dB/100 m] or dB per 100 ft (dB/100 ft) by the length of the cable used.

Each connector used between antenna and receiver will add a few tenth of a dB of attenuation, the same applies to adapter e.g. those used to convert from one connector type to another. The actual attenuation of connectors varies with the connector type (e.g. simple coax connector, threaded types like N, TNC, SMA, or Bayonet types like C, BNC, ) or for how many connections they are rated, some versions for as low as only 5 connections.

Any gain obtained by an antenna is easily offset to a large degree by the attenuation of a few meter of high loss transmission lines on higher frequencies (e.g. thin coax like RG-174)

For example you connect a monopole optimized for 1000 MHz with about 5.25 dBi via 5 m of coaxcable to a receiver

If you use RG-174 from Fairview which specifies the max. frequency as 1000 MHz and an attenuation of 1.0499 dB/m (https://www.fairviewmicrowave.com/image ... /RG174.pdf ), the cable loss for 5 m of their RG-174 would be 5.25 dB and compensate all of the antenna gain.

If you use a more rigid, but low loss ¼ inch Cellflex coax cable instead e.g. LCF14-50JFN from RFS which specifies the attenuation of 0.139 dB/m the attenuation is reduced to just 0,69 dB for 5 m LCF14-50JFN, leaving you with an antenna gain of 4.5 dBi.

Therefore it is often more sensible to use coax cable that provides low loss at the frequency you want to receive at, than trying to increase antenna gain to compensate cable loss.

Please note: unfortunately not all coax cable of the same type (e.g. RG-58) will have similar loss. There are also cheap knock off copies, with just a few strands of wire as shield without providing any real shielding, and do not reach the low attenuation by brand name manufacturers.

Guidelines for choosing coax cable for use >1000 MHz

Posted: Wed Oct 09, 2019 8:17 pm
by snn47
Guidelines for choosing coax cable for use >1000 MHz possibly not complete, but that should suffice for a start:

Know which impedance receiver has and you know what impedance your coax cable and connector should have

Decide if you desire low loss, max. shielding or both
While low loss per dB/m (or db/ft) is desirable, if you receive interference through a bad braid shield of a cable, despite using a filter, low loss won't make you happy either.

There is no coax cable that is cheap, provides low loss, max. shielding and stable impedance,
but you can optimize


The price will differ with availability and country, and should get cheaper the more you buy
Unless you can buy a ring of e.g. 100m and share the cost with a friend, you will pay more per meter.

It's not just the coax cable price, but take into account the cost for the connectors and possibly adapter
Coax connector for a cheap coax cable of uncommon dimensions can be more costly.


Now that you know what you want, you have to choose the best cable for you

Buy only name brands coax cable

Inform yourself which coax cables, from which manufacturer and the price are available, where you live
check the cost for coax connectors and adapters


For some cable types like e.g. "RG-58" type, you need also the letters following the RG-58 or even the serial number to be able to identify which cable you will get.
If you check the Belden datasheets you will find several RG-58 type coax-cable, which differ in attenuation, shielding and therefore price.
Note: Better shielded coax cables, double shielded, with PTFE isolation, are also available in the RG-58 diameter, but are as consequence more expensive.

download the data sheets from the manufacturer and compare the attenuation and shielding factor and decide which brand name cable you can afford.

Mechanical considerations

1. bending radius, make sure that you can easily mount the cable
The best cable is not worth spending the money if you damage the cable by bending it to much. Because the min. bending radius was not observed, several hundred meter of expensive 1" coax cable had to be replaced

2. mechanical stress on coax connectors, cable, adapter and receiver
you can get SMA adapter even for thick coax-cable 1/4" or much larger, but remeber that you have to avoid mechanical force and continued stress on all component.

Attenuation and shielding
I would not call/consider any coax that has the diameter of the 50 Ohm RG-174 type coax a good quality thin cable for use at 1 GHz.
Belden identifies the attenuation in the data sheet to be 1.08 dB/m @1000 MHz loss. If you use a cable with PTFE it's down to ~0.8 dB/m @1000 MHz.
The minimum cable for a few meter should be a brand name e.g. 9201 Coax - RG-58/U Type , Belden for loss 0.476 dB/m @1000 MHz, 20 AWG solid .033" bare copper conductor, polyethylene insulation, bare copper braid shield (80% coverage), PVC jacket

While also being called RG-58 type, the 7807A Coax - RG-58 Type has less than 0.3 dB/m @900 MHz, a 17 AWG solid .044" bare copper conductor, gas-injected foam HDPE insulation, Duofoil® (100% coverage) + tinned copper braid shield (95% coverage), polyethylene jacket.

Then there are fully shielded cable, like cellflex, that have a solid cooper shielding and provide therefore 100 % shielding. In addition to the higher price, they should not be moved or bend more than a few times.

As for 75 ohm CATV-cable I bought a 100 m ring double shielded WISI MK-90 for about 50 Euro at the time. The newer MK-91 has now triple shielding and ~0.2 dB/m @1000 MHz attenuation, which is mechanically not as stable for continued moving/bending, and has to be impedance matched to 50 ohm receiver antenna input.

Power rating
While not of interest for receive only, the power handling capability of a coax cable is something to consider if you also transmit.