L-Band receive antenna (960 MHz to 1215 MHz)

Discuss commercial and home made antennas.
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L-Band receive antenna (960 MHz to 1215 MHz)

Post by snn47 » Sat Jan 21, 2017 8:25 am

continuation of discussion started at http://www.rtl-sdr.com/radio-for-everyo ... /#comments

I understand and appreciate your aim “keep it simple” and that it is diffuicult to achieve.

What I was trying to point out that guidance should therefore be the simplest antenna, including transmission line and connectors that can be reproduced. I remember a few fights, when manufacturer went to court, because another HAM (engineer) did thorough tests on scaled down models on his home made antenna test range. The manufacturer /designer however didn’t like the measured results.

As for the design I know of a few commercial ground planes, mainly for VHF/UHF, I never saw such a design without impedance matching. Do you have a reference to a manufacterer or describing such a ½ wave ground plane you can direct me to.

While in principle larger antennas like ½ wave, 5/8 wave or stacked ½ wave dipoles can achieve higher gain, they will not due to the short wavelength. In the L-band a few mm in length or diameter will decide whether you achieve real gain with the desired gain and pattern or have only placebo gain and pattern for your peace of mind. In most cases a ¼ wave macaroni cooked in enough salt water mounted above a large counterpoise should provide a better pattern and gain then the antenna designs provided in many posts .

¼ Blade antennas, except for the occasional flush mount, have not been replaced so far in aviation for aircraft use so far since introduced for DME in 1949. Even ¼ blade antenna patterns are highly susceptible to all objects protruding within a few wavelength. Test results from using scaled down models during Mode S development for FAA are documented in ATC reports for at the MIT website.

With difference in cable loss I was pointing to the difference in cable loss between cable used at at the ½ wave cable design and the ones attached to the magnetic mounts, which for cost reasons will not use any of the available “low” loss thin cable. Any cheap double shielded cable for distribution of SAT-TV between 900 to 2000 MHz is therefor better then >1 dB/m for the thin cable.

Please note: the L-Band covers by definition a much wider range than identified in the title, but that's the range aircraft operate in.
Last edited by snn47 on Mon Feb 06, 2017 9:40 pm, edited 2 times in total.

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Re: L-Band receive antenna

Post by radioforeveryone.com » Sat Jan 21, 2017 6:43 pm


Thanks for your words.
I'll test that antenna in real life. I do know that impedance matching, proper measurements etc can and will increase performance, however, that antenna works quite well.
I'll run a test against three other antennas to demonstrate its capabilities. I'm a great believer in the KISS principle, even if sacrificing ultimate receive performance on the altar of ease of construction, material accessibility etc.
If you go on my website and read the easy Outernet antenna post, you'll see that it actually works. I haven't invented hot water, simply applied known principles and antenna results to a task, after reading PhD level pdfs on helical antenna construction.
I also read quite a lot, and a thread I see in almost all publications, from ARRL to antenna theory, is that authors and a large number of folks rely on what has been done before. "In principle" is great, as theory provides a solid foundation to what I'm trying to achieve, but the vast majority of readers would close a window the very second I started to discuss radiation patterns, lobes and SWR.
My aim with the webpage is to provide an entry world into the world of radio, but without excessive technical details - please read my manifesto on the page for a longer version to see what I mean.
Eventually, I think we're both on the same side, as we both love radio - that's clear from your level of expertise in this subject matter. If you wish to share your knowledge, I'm more than happy to provide access to the website, because I'm sure readers would be delighted to read about ADS-B antennas.
Many thanks for your comment,


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Re: L-Band receive antenna

Post by snn47 » Sat Jan 21, 2017 8:01 pm

Since I first mentioned the L-Band Blade antenna used on aircraft, I started to look for information on blade antenna design without much success. The design is old <1948, I couldn’t find a patent, nor anything on the design.
The only cutaway picture of an L-Band Blade antenna showing the design I know is from a larger picture showing the DME interrogator built by Federal Telecommunication Laboratories, that’s ‘s the DME specified by ICAO in 1949, not today’s DME/N
They needed to achieve a large bandwidth of 963.5 to 986 MHz for interrogation and 1188.5 to 1211.0 for receive of the ground replies. Instead of a thin wire, but a flat spade to widen the bandwidth was used, like a fal monoconical antena on the large counterpoise.
_#DME-Blade_[TDR-114_CAA1950].png (59.29 KiB) Viewed 12561 times
I don’t know, if the tube below, which was when mounted within the fuselage , was providing some matching or not, nor if today’s L-Band Blade antenna designs differ much from this design. Maybe someone else knows more and will share it with us or someone owning a L-Band Blade antenna can try to take a picture, while the Blade antenna is x-rayed, when his carry on lockage is checked at an airport.
_#DME-Blade_DME-CAA-1949.png (194.68 KiB) Viewed 12561 times
DME, SSR and non directional ACAS antennas are interchangeable by definition.

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Re: L-Band receive antenna

Post by snn47 » Sat Jan 21, 2017 8:16 pm

Sorry I, I should have used diamond antennae instead of spade, for a lack of the correct designation in english, but I could not correct my post. Everytime I tried I could not save it, because I received the message "This message was flagged as spam and has been denied."
On second thought would flat monoconical antenna on a counterpoise be a better description.

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Re: L-Band receive antenna

Post by rtlsdrblog » Sun Jan 22, 2017 7:25 am

The spam protection is set quite high for the first two posts, but after that it starts to get less aggressive.

Interesting design on the aircraft. I think 1/4 wave GP antennas are the most popular because they will always work well with a forgiving radiation pattern, have a good easy impedance match, and are easy to build, and are nice and small for ADS-B.

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Re: L-Band receive antenna

Post by snn47 » Sun Jan 22, 2017 10:04 am

That is the L-Band Blade in use 1948 and not an ILS-GP antenna. While today's Blade antennas are in use also on lower bands like e.g. for VHF- and UHF-Com. For ILS-GP and ILS-LLZ the folded V-dipole or alford loopd design was commonly used in 1948.
Today there are many non L-Band- or multi-band-Blade-antenna designs/patent registrations that you can find.
While there are normally patent registrations for everything, down to the last nut and bolt if you can file for a patent, I wonder why my search did not find anything on L-Band Blade antennas.

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Re: L-Band receive antenna

Post by snn47 » Tue Jan 31, 2017 11:37 am

If you look for NARCO transponder (e.g. AT-155), you'll find the cheaper alternative antenna to a blade antenna used by GA (see second picture).
It is basically a 1/4 stub and requires a conductive counterpoise/groundplane. The small ball on top reduced the physical length a bit,but will have also impact on the antennas impedance and will require matching if the antenna gets much shorter than 1/4 wave due the high capacitive loading.

For aircraft without a metallic fuselage, the recommendation from NARCO is
B. Non-Metallic Mounting Surface
Aircraft with fabric, wood, or fiberglass fuselage covering must have a metal ground plane with a 6" (152.4
mm) minimum radius. This could be as simple as aluminum foil cemented inside wood or stiff fiberglass
skin, or a doubler plate on a fabric covered aircraft. ... Antenna mounting
hardware must electrically connect the ground plane to the antenna.
The antennas are mounted on the bottom of the aircraft

For home use invert the 1/4 wave wire, add four 1/4 wave pieces of metall rod connected in a X fromation (see picture) ro the outer shielding of the cable, and it will make it a 1/4 ground plane. The actual length of the 1/4 wave stubs will be shorter than the calculated value . The the thicker the wire is the shorter it has to be to stay in resonance.
0.25-GP.png (2.84 KiB) Viewed 12343 times
_#ant-air-SSR-i_AT-150_Narco.png (14.19 KiB) Viewed 12343 times

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Re: L-Band receive antenna

Post by jmcc » Sat Feb 04, 2017 1:50 pm

snn47 wrote:With difference in cable loss I was pointing to the difference in cable loss between cable used at at the ½ wave cable design and the ones attached to the magnetic mounts, which for cost reasons will not use any of the available “low” loss thin cable. Any cheap double shielded cable for distribution of SAT-TV between 900 to 2000 MHz is therefor better then >1 dB/m for the thin cable.
A lot of the good quality thin cable such as RG174 etc was not really intended to be used with long runs. The SAT-TV cable would be lower loss but again, the quality can vary. The signal levels with satellite TV LNBs and receivers are often higher than with off-air signals. The domestic SAT-TV cable might be fine for small runs but there are more expensive and better performing SAT-TV cable types available via trade suppliers.

There's actually a very simple (non-technical and not really good engineering) way to illustrate the effect of resonance with a mag-mount antenna and PlanePlotter. Use the received off-air message rate window to see the effect of lengthening or shortening the telescopic antenna. It is also possible to use SDR# or any other SDR software to visually represent the effects. You can also use the same methods to see the difference between various types of cable.

There's really no substitute for good quality coaxial cable for this kind of experimenting as it removes some of the uncertainty. The problem for hobbyists (rather than HAMs and those in the RF engineering business) is that good quality cable can be very expensive. It can be the difference between buying a TV extension kit with ten /metres of cheap cable for about $10 or 10 Euro or paying a few Euro or Dollars per metre of good quality cable. And breaking down their only mag-mount antenna might be a bit off-putting for most people. With broadcast signals, poor cable quality won't be apparent unless the signal is poor but a lot of the signals that people want to receive with SDRs are rarely at broadcast levels.


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Guidelines for choosing coaxcable for use >1000 MHz

Post by snn47 » Mon Feb 06, 2017 9:37 pm

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.

Aussie Susan
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Re: L-Band receive antenna (960 MHz to 1215 MHz)

Post by Aussie Susan » Tue Feb 07, 2017 2:21 am

Excellent advice.
Also consider cables that are labelled LMRnnn. I use LMR400 to connect to my 23cm antenna (for amateur radio purposes) and I've used LMR200 (slightly more lossy but cheaper) in the past.

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