Antennas - To gain or not to gain, that's the question

[Smokey]

The internet, especially ADS-B forums, are full of ideas about antenna's for 1090MHz, although many refer to adaptations from other applications & frequencies.

It bothers me that nobody gives any background info or theory that backs or explains their design. One popular design is this one http://www.tech-software.net/1090_ant_02.JPG (and there are 2 more with different dimensions ) – WHY ?

I have read somewhere,  someone giving the advice "just follow the design closely" with reference to antenna's.  I WOULD like to know how the designer got to that length. If all the designs were very "close", one could conclude the difference is purely due to rounding etc. But that is not the case. Did the designer have his invention coupled to a spectrum analyser & tuned it ?

Just some background of my experience so far, maybe that will explain my questions below

I started in Nov 2012, borrowed a dongle 4000 (awaiting my 820) and bought a professional collinear ant of roughly US$130 (as used by my mate on his Kinetic box), mounted the ant 12m AGL with 15m of LMR-195 cable, N-Type on antenna side, BNC  male on receiver side with a pigtail (BNC to PAL): Plots show a MAX 75nm. Thought it was the dongle, so when I got the 820, tried it again and distance went up to 100nm – still disappointing after reading you guys in the UK get 200nm plus.

This same mate of mine had made an antenna 2 years previously, based on the top link, never used it, so thought he will bring it to me to try out.

Whereas the antenna as per the link, lengths are 136mm/205mm/190mm compared to the my mates version 137/201/192 – not much I hear you say. BUT his lengths are measured from start to "bottom of 1st loop", then 8mm from "bottom of 1st loop to top of 1st loop", the 2nd element from "top of 1st loop" to "bottom of 2nd loop" – again 8mm for the loop's "vertical" or rise length before starting the measurement for the 2nd element. So, If one then takes the advise as posted elsewhere, use the center of the conductor as reference, then add 4mm to each element for my mate's version, now it becomes 141/205/197. The other difference compared to the link's version is, he soldered 4 ½ wave, 45deg ground legs on

This bare 3mm Cu element antenna gave me on the same setup (Dongle 820 but changed the PAL connector to MCX) in excess of 200nm (peak plotted @ 219nm) pic attached. I then mounted the antenna in a 50mm PVC pipe, capped the top and sealed the bottom with hot glue. I saw a drop in distance to just over 200nm (peak plotted @ 201nm) I noted that weather conditions were very similar on each of the two 20hrs tests.

Now we knew he had a good design & I tried to replicate it ... yea right – have not managed to match it in 2 & ½ months. I then tried various versions, based on designer numbers & even more based on theory alone, some ¾ wave, some with VF taken into account etc – still no match yet.

Then I got an email from him with a link to a very simple (and funny) tutor to build your own ¼ wave mono-pole with 8 x 45deg ground legs. This little number performed so well in my office, I then moved it outside the window on a 2m pole & got even better results. Next step was mounting it on the roof @ 12m. Results were good, matching the capsulated beast of my mate (200nm). See pic

The conclusion I came to (apart from thinking I should try to steal his antenna back) was that none of my collinear antennas were producing any gain, as the ¼ wave monopole was a zero gain design and performed better than any collinear design (my mates version excluded)

Just to give an indication of how good his version is ... On his Kinetc box with commercial antenna v.s the NooElec 820 dongle, the box seem to have very little advantage – seem to remember him quoting  "the Box drops an AC around a minute after the dongle". He then connected an amplifier (not on top at the antenna) just before his dongle. The result ... the dongle gain 20nm on the box. Next weekend we will install the amp on top as it should be ... and lets see what happens.

On that point – AMP ... I have promised myself I will not cut any corners by installing an AMP to compensate for a poor antenna design, so till I build an antenna that actually gives gain, I will NOT buy an AMP. And for those asking "whats wrong with 200nm?", if one CAN get more out of an antenna, then that IS what I WANT

( Keep in mind, I do not have access to a spectrum analyser, so every new antenna I make, needs to be evaluated on the roof top over a week and then we hope that the weather & atmospheric conditions are similar for comparison purposes )

I would like to get answers to the following:

1.   What should the theoretical lengths be for the 3 elements & the loops
2.   What VF (if any) should I use e.g. for bare Cu rod 3mm dia in free space vs stuck in a 50mm PVC pipe
3.   What point/s do one use for measurements -  outer/centre etc
i.e. should the 2nd element length be measured from top & bottom of the loops or centre to centre ?
4.   What material would be ideal
5.   What should the diameter  of the radials be
6.   Ground plane – disk or 45deg "legs" / How many legs 4, 8, 16, Mesh /  Length of the legs / Diameter in case of a disk
7.   Encapsulate or open / If Open, what protective layer would be OK / If encapsulated, may the elements / loops touch the side of e.g. the PVC pipe

I hope all the antenna boffs & radio hams with the answers are willing to assist


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[Chris-G0WTZ]

Antenna theory is very complex. I sell the Panorama antennas and they design them and test them with full back up documentation.They are one of the few antenna manufacturers in the UK to have an antenna test chamber. Take a look here for details. http://www.antennatesting.com/chamber.shtml

I have just received sample of a hi gain colinear which has some good results. When I asked for specification they say it is a text book multi array colinear.

If you can find it an excellent book to understand how antennae work is the 2 metre antenna handbook by FC Judd

Hope that helps a bit

Regards

[threemilesfinal]

so you want to know antenna theory eh?

as mentioned, it is very interesting and complex. i'm currently working on one for my HF rig.

to help you out, here's a link to the ARRL Antenna Book. all the science and theory for antenna design can be found in here.

https://docs.google.com/file/d/0By0RSii0GE_Zak5XcW5WSmNBZjg/edit?usp=sharing

cheers mate!

[edgy]

The thing about a colinear is that the angle of radiation is not zero.  Depending on tolerances, it may be a very large number of degrees.  A colinear needs no ground-plane.  Many Hams mount a colinear upside down to use as a repeater antenna (high mounted, it then looks down).

A dongle has very limited dynamic range, so it would be excellent for a short range no gain antenna (even no antenna).  Multiple dongles is better than one dongle with a high gain antenna.

High gain antenna, means the donut shape of the pattern is squashed more and more.  Thus a high gain colinear may be pointing at Venus rather than the horizon.

If you want to use a dongle, you can increase the gain, as you narrow the beamwidth.  Eight dongles with eight directional antennas (no backbeam) would be just the ticket.  A good antenna with no backbeam can be made from PC board material into a waveguide cut for L-Band  (WR-770 or 7.7 in x 3.385 in). Make it about a foot long, and put a feed 1/4 wavelength from the closed end.

Film at 11...

[E:V:A]

...
I would like to get answers to the following:

1.   What should the theoretical lengths be for the 3 elements & the loops
2.   What VF (if any) should I use e.g. for bare Cu rod 3mm dia in free space vs stuck in a 50mm PVC pipe
3.   What point/s do one use for measurements -  outer/centre etc
i.e. should the 2nd element length be measured from top & bottom of the loops or centre to centre ?
4.   What material would be ideal
5.   What should the diameter  of the radials be
6.   Ground plane – disk or 45deg "legs" / How many legs 4, 8, 16, Mesh /  Length of the legs / Diameter in case of a disk
7.   Encapsulate or open / If Open, what protective layer would be OK / If encapsulated, may the elements / loops touch the side of e.g. the PVC pipe
...

Hi Smokey,
Did you get any smarter with this? I have the exact same questions. The various stacked Omnidirectional's have widely different designs, and equally widely different results. I'm trying to find more optimal design than those... I do have some answers for some of these questions.

A1: L = c/1090

Code Select Expand


        L               = 275.03
        L/4             = 68.76
        L/2             = 137.52  (L1)
        3L/4            = 206.27  (L2,L3)

        S turns         = <see below>
        S length        = L/4
        S diameter      = 21.87 @ 1 turn
        S pitch         = -
        S chirality     = RH (S1=S2)

        Wire diameter   = 2.76 mm2 ("A6")
        Radials         = 4
        Radial lengths  = L/4
        Radial angle    = ~37 ?

A2: Depend on the wire diameter:

VF[Cu(insulated)]   = 0.93 - 0.98
VF[Cu(bare)]      = ~ 0.98 ?

"Velocity of see through PVC insulation on 2.5 mm square wire was .95.
Velocity factor of same wire with bleu stripe was .94
same wire with red insulation PVC was .92.
same thickniss wire with black carbon coloured PVC coating was .86.
bare wire was .97 ."

A3: center
A4: Material for what?
A5: Depend on number of radials, but for N<16,  L > 0.1 wavelength is pointless.
A6: Depend if you want to use it for impedance matching. But the side effect is that this also affect the "lobes" angle/altitude (dBi). I've seen 30-45 degrees for 50 Ohm matching...
A7: Avoiding touching the sides are always better, but probably no noticeable effect, unless thunderstorm or high winds (static).


Now if someone could answer the following questions:

a) Are there any better tables or way to calculate the "effect of conductor diameter" available? (I've used the table in ARRL Fig.21.3, but it is very small and I have to extrapolate quite a bit.)

b) How do I calculate the "compensation for end capacitance" on the last element, (3/4 lambda) L3?

c) How does opposing "handedness" (chirality) of the phase stub loops affect the design? Some designs have both right handed loops, and another have opposite. 

d) I find it strange that these designs are using two 3/4L radiating elements (L2,L3) instead of half wave? From what I understood of stacked monopole antennas, it seem that if part of the other phase of the wave is present on the whole radiator, then there will be wave cancelling as seen from a distance. That's why we have the phase stubs... this lead to the next question.

e) It seem that some books/people use phase stubs with quarter wave length (L/4), but I though we have to compensate a half wave!? For example, look at this screwy description: http://electriciantraining.tpub.com/14182/css/14182_193.htm

I'd love to hear what you antenna experts have to say...

Thanks in advance.