Author Topic: Tuning the DVB Antenna for 1090MHz Specific reception  (Read 31031 times)

Bill Maxwell

Re: Tuning the DVB Antenna for 1090MHz Specific reception
« Reply #45 on: February 18, 2018, 05:22:48 am »
In theory a  half-wave antenna would be a better performer for more distant signals, given its radiation pattern, although that is more important for transmitting than receiving and given the strength of ADSB signals, the difference would probably be negligible.

Half-wave antennas also less reliant on ground, although we shouldn't go as far as to declare them to be ground independent, as some manufacturers claim.

One other significant difference though is the feedpoint impedance. The quarter wave is a relatively low impedance feedpoint device, hence our ability to fed it happily through 50 ohm coax cable. Not so the half-wave. It is a high impedance feedpoint device and should really be connected to the coax via an impedance matching circuit, such as a parallel combination of a coil and a capacitor, tuned to the frequency of operation. That tuned circuit, often called a tank, has a low impedance at the bottom of the pair but a high impedance at its top. That characteristic of a parallel tuned circuit makes it easy to connect the antenna to the top and the 50 ohm coax to the bottom.

in terms of tuning the antenna to resonance, 4mm at 1090MHz is a significant difference in length.

exfirepro

Re: Tuning the DVB Antenna for 1090MHz Specific reception
« Reply #46 on: February 18, 2018, 09:39:01 am »
Hi Bill,

Long time no speak! How are things out in Oz? Nice to hear from you again. I had read Rog’s post yesterday morning but was just about to head up to Perth (UK not WA) to try to help sort out some PAW probs in a couple of RVs, so had to leave it.

I knew there was a technical reason to do with voltage and impedance at feed points and was racking my brain on the drive up but couldn’t dig out the details, so thank you for a very clear and accurate explanation.

Best Regards as always

Peter
« Last Edit: February 18, 2018, 09:41:53 am by exfirepro »

Bill Maxwell

Re: Tuning the DVB Antenna for 1090MHz Specific reception
« Reply #47 on: February 18, 2018, 10:13:54 pm »
G'day Peter

I still monitor the Pilot Aware scene, even though I can't use it here in Oz. Speaking of which, I would not have been confused about your heading up to Perth in Scotland, rather than Perth in Western Australia. I might though have been confused in to thinking that you were maybe heading  to Perth in Tasmania, which is older settlement that that young upstart village over in the west. In that case, since it is only 11 kms up the highway from here, I would have invited you to drop in for coffee and even invited you to use my strip :)

That said, you would probably have written that you were heading down to Perth, given our inverse orientation.

Cheers
Bill

PaulSS

Re: Tuning the DVB Antenna for 1090MHz Specific reception
« Reply #48 on: February 19, 2018, 01:42:52 am »
Quote
that young upstart village over in the west.

HEY, some of us live in that 'village'  ;D

Mind you, if this village has 680 times the population of the Perth in 'ladies parts' I don't know what that conurbation would be called....a 'hovel'?  ;)

Is that your strip I can see in Powranna (no village), close to the highway? That looks a great place for an aircraft  :)

Bill Maxwell

Re: Tuning the DVB Antenna for 1090MHz Specific reception
« Reply #49 on: February 19, 2018, 11:24:21 pm »
Yes Paul, that would most likely be my strip, aligned 31/ 13 and right next to the highway. There are two other strips in Powranna, one on Powranna Road but aligned more east/west and an old one on Symmons Plains property just to my north and on the other side of the highway, behind the motor racetrack. The latter is aligned 32/14, which shares my advantage of nicely suiting our prevailing north westerly winds. It is used very occasionally though, except for model aircraft that are flown there most Sunday mornings.

The local crop dusting company likes to use our strip when dropping fertiliser granules etc on the neighbouring croplands. We had a Thrush operating off the strip a month ago. Nothing like the sound of a 800shp turbo running up in your own front yard!

The only things to watch out for here are the A.C. Mains (3 phase) feedlines that run to the house from the pole across the highway on the southern approach and the what might be politely termed "close proximity" of Class D airspace for Launceston ( and for the non-Australian resident readers, we are not talking of the one in Cornwall). Luckily, the Launceston Airport isn't what you could really consider as busy.

Paul_Sengupta

Re: Tuning the DVB Antenna for 1090MHz Specific reception
« Reply #50 on: February 20, 2018, 03:34:54 am »
The point of cutting down the antenna isn't really to tune it, but rather to make it fit in a tighter location while still retaining a fairly good receive ability. The impedance mismatch shouldn't matter too much for receive.

Samprice11

Re: Tuning the DVB Antenna for 1090MHz Specific reception
« Reply #51 on: September 29, 2018, 04:05:05 pm »
Hi all,
just to add a little more info on this subject, I've just done a 'before and after' check by using a shortened DVB-T 'whip' aerial as suggested in the original post. The screenshot show the increased detection range achieved by shortening the aerial to 50mm.

For info, my reason for doing so was mainly to aid installation into a pa28 rather than increased detection but it's a nice 'bonus' to achieve as well as making the system more compact for installation.

Thanks,
Sam
« Last Edit: September 29, 2018, 04:08:39 pm by Samprice11 »

exfirepro

Re: Tuning the DVB Antenna for 1090MHz Specific reception
« Reply #52 on: September 30, 2018, 10:12:30 pm »
Hi Sam,

An antenna carefully tuned to the correct frequency will always be more efficient than one of random length, hence the effects you are seeing. Good to see ‘physical evidence’ to support the theory though and the shorter length is definitely a bonus when it comes to achieving an effective fit. Well done.

Best Regards

Peter

Nobubbles63

Re: Tuning the DVB Antenna for 1090MHz Specific reception
« Reply #53 on: November 06, 2018, 03:49:10 pm »
Having read the posts with interest and having shortened the cable, can I please clarify the point at which the aerial mast will be cut. I have the 19mm exposed coax and inner aerial post and therefore need to cut the mast to 50mm.
Therefore, do I understand correctly, if I unscrew the aerial and lay it down, that when cut the entire bit left will be 50mm? This will obviously include the screw-on post section so tip to tip 50mm, with the screw-on section being 10mm of that, leaving 40mm of wire?
Or, is it 50mm of the thin “wire” aerial on top of the screw on post?
Have seen the earlier confusion with where to cut.
Thanks
Derek

exfirepro

Re: Tuning the DVB Antenna for 1090MHz Specific reception
« Reply #54 on: November 06, 2018, 04:08:07 pm »
Hi Derek,

Overall length needs to be 69mm, so you are correct - cut the whip to 50mm from bottom of the female coupler - I find a good strong pair of ‘side-cutters’ work well, but watch out for the tip heading off at speed (I usually cover it with a cloth while cutting to catch the cut end). The plastic end cap can be gently persuaded off the waste bit with a bit of gentle heat from a heat gun or similar, then fixed back on the new tip with a spot of superglue (hot glue also works).

As you will have read, the overall length is fairly ‘non-critical’ by the way as the antenna has already been working as receive only.

Regards

Peter

Nobubbles63

Re: Tuning the DVB Antenna for 1090MHz Specific reception
« Reply #55 on: November 06, 2018, 04:32:22 pm »
Many thanks Peter, understood.
I was just a bit nervous as the posted pictures in this discussion were looking a bit longer than what mine was going to look like...must just be the perpsective of the pics. Right....50mm from the base of the screw end it is.
Regards
Derek

exfirepro

Re: Tuning the DVB Antenna for 1090MHz Specific reception
« Reply #56 on: November 06, 2018, 04:52:19 pm »
Most times I’m cutting them down, it’s to fit the PAW on a coaming against a windscreen, so as the length isn’t critical, I often just cut the whip ‘in half’ by eye - it still works fine for traffic awareness, bearing in mind we can only see aircraft a couple of miles away at best.

Regards

Peter