best antenna

[gregy]

and the other challenge...
at these frequencies (1090 MHz) the lead length and other physical characteristics of variable cap will likely introduce
inductance components ... thus not sure how true to the labelled capacitance they would be at 1090 ...

[abcd567]

Nice try ab cd. I await your next brain wave.

I have not given up yet. Another brain wave. I can achieve lower capacitance by putting several small capacitances in series.

If two capacitors C1 & C2 are put in series, the resultant Capacitance C is given by formula:

1/C = 1/C1 + 1/C2 = (C1 + C2 )/C1 x C2
or
C = C1 x C2 / (C1 + C2)

if I put a fixed capacitors of 2pF and a trimmer of 2.5pF minimum value, I get
Minimum Value of trimmer = 2 x 2.5 / (2 + 2.5) = 1.11 pF



If three capacitors C1, C2 & C3 are put in series, the resultant Capacitance C is given by formula:

1/C = 1/C1 + 1/C2 + 1/C3 = (C1 x C2 + C2 x C3 + C3 x C1)/C1 x C2 x C3
or
C = C1 x C2 xC3 / (C1 x C2 + C2 x C3 + C3 x C1)

if I put 2 fixed capacitors of 2pF and a trimmer of 2.5pF minimum value, I get
Minimum Value of trimmer = 2 x 2 x 2.5 / (2x2 + 2 x 2.5 + 2.5 x 2) = 0.72 pF

If I add 3 capacitors of 2 pF inseries with a 2.5 minimum value trimmer..... well it will be sure lesser ...between 0.5 & 0.6 ... don't want to calculate further, already have a headache

[gregy]

16.6nH coil
air cored, 4mm diameter (internal), 2 turns, spaced about 2mm overall length
(inductance can be tuned by squezzing or expanding the coil length)

[gregy]

sorry, read your chart wrong
17.9nH
air cored, 4.5mm diamter (coil internal dia). 2 turns, length 2.45mm ( note its quite sensitive to the length)

[abcd567]

sorry, read your chart wrong
17.9nH
air cored, 4.5mm diamter (coil internal dia). 2 turns, length 2.45mm ( note its quite sensitive to the length)

Thanks.

Yes, you are right. The coil inductance is very sensitive to length & dia of coil. That is why I wanted to avoid using it for DIY situations, as the average DIY enthusiast's skill level is not very high.

I have already worked out coil dimensions couple of days ago, using a website calculator. Thanks for help anyway.
Coil wound over 1/4 inch (6.4 mm) dia drill bit, 2 turns, coil length 9mm gives an inductance of 17.84 nH.

Website address: http://wcalc.sourceforge.net/cgi-bin/air_coil.cgi

[abcd567]

16.6nH coil
air cored, 4mm diameter (internal), 2 turns, spaced about 2mm overall length
(inductance can be tuned by squezzing or expanding the coil length)

Yes, you are right. The coil inductance changes by changing length. Expanding reduces inductance, squeezing increases it.

This phenomenon was used in the old fashioned non-digital AM/FM Radios to adjust the front end antenna & oscillator circuits.

The FM dial's frequency adjustment was achieved during final stage of manufacture by expanding/compressing the coils.

[abcd567]

and the other challenge...
at these frequencies (1090 MHz) the lead length and other physical characteristics of variable cap will likely introduce
inductance components ... thus not sure how true to the labelled capacitance they would be at 1090 ...

You are absolutely right.

As a starting point, we have to ignore lead indutances & stray capacitances, and assume the labelled value only, then adjust the trimmers & coil to get best results, using software "rtl1090" 's air data indicator lights at bottom left to judge the output. For those lucky ones having access to a VNA, adjustment can be much easier & accurate.

[abcd567]

I have little hope that antenna impedance matching network (T, Pi, L, Series, or Parallel compensation) with adjustable controls, will be successful in DIY situations due to extremely low values of Capacitance & inductance required for matching network.

Variable Capacitors for very low values are not available. Variable inductor can be made by winding an air core coil, but due to very few turns, accurate adjustments will be very difficult by expanding/squeezing the coil.

Impedance matching network may succeed with those lucky ones who have access to a VNA (Vector Network Analyzer).

[abcd567]

There are GHz Active Inductors & Active Capacitors using CMOS and MESFET negative resistance. Their values can be varied by applied voltage/current by a potentiometer (like volume control knobs in radios & audio players). Maybe we can use these for impedance matching of CoCo.

At this stage, this is just an idea, needs detailed study to find out feasibility & practicability.

(a) 1 GHz CMOS Band-pass Filter Design Using an Active Inductor and Capacitor

(b) Active Inductor and Capacitor for DCS receiver band (1.71GHz – 1.785GHz) using multi-MESFET Negative Resistance Circuit

(c) A New Active Inductor and Its Application to Wide Tuning Range LC Oscillator

(d) CMOS Active Inductors and Transformers: Principle, Implementation, and Applications - by Fei Yuan

[abcd567]

No input from forum members on CoCo impedance Matching/Tuning equipment/techniques???
Many forum members are Amateur Radio Operators, some are long-timers as well, and sure have good experience on this issue. their input can be very valuable.

[abcd567]

In DIY environment, CoCos are rarely made accurate enough to give good performance.
Although inaccuracies affect Gain & SWR both, but the Gain is relatively less affected by errors than the SWR, which changes profoundly with changes in length of element.

What I am aiming at is a "Magic Box" which can be inserted at antenna feed point, and which should be capable of compensating the affect of these inaccuracies.

My objective is to find an impedance matching / SWR reduction method which can achieve this by simply turning one or two knobs/adjustments and without use of any equipment such as Network Analyser (VNA) or SWR Meter etc.

Any ideas / sugesstions?

[abcd567]

Simulation results for CoCo made of RG6 Coax (FPE insulation, 0.83 VF, element length=114mm, half-element length=57mm)
Proves CoCo is a high impedance high SWR antenna.
Adding a shorted 1/2 element & a whip could not bring SWR below 3

1.	8-Element	SWR = 3.4	Gain = 8.55 dBi
2.	8.5-Element, Shorted Top	SWR = 2.92	Gain = 8.93 dBi
3.	8.5-Element, Shorted Top+Whip	SWR = 3.57	Gain = 8.85 dBi

8-element
Gain-Pattern Swr-CoCo 8 Element RG6 FPE VF 0.83.PNG

8.5-element shorted top
Gain-Pattern Swr-CoCo 8.5 Element Shorted Top RG6 FPE VF 0.83.PNG

8.5-element shorted top + whip
Gain-Pattern Swr-CoCo 8.5 Element Shorted Top + Whip RG6 FPE VF 0.83.PNG

[Rooster]

No input from forum members on CoCo impedance Matching/Tuning equipment/techniques???
Many forum members are Amateur Radio Operators, some are long-timers as well, and sure have good experience on this issue. their input can be very valuable.

I said this before, and Ill say it again...

You are using a VERY large hammer to crack the nut !
I'd bet that if you took a straw poll, 99% of the forum users couldn't care less about the "rocket science"... they only want to knock up a working antenna.
Matching feeder to antenna at these frequencies requires an accuracy few could attain, so is basically a waste of effort - without the correct test equipment totally pointless.

When I first got my TV dongle, I made a coco out of spare satellite cable... no idea of specs whatsoever ! 11cm a section.... fed with an old tv coax feed into the shack (again, zero maths)

Compared to a commercially built active scanner antenna (same feeder) the coco provides more than 10dB gain over the scanner antenna.

So YES you can lash up a good antenna with nothing more than a tape measure.

[peterhr]

This is exactly why - for newbies - I suggest making up a dipole and to tie it to a gallows made of bamboo cane.

The cost is just some old coax (they might have in the shed), some cane (from the garden)

Fix it to the spouting / guttering at the roof edge so it's as high up as possible and vertical

The only things to be careful of are not a long down lead (5m or so is ok), and getting decent connectors to adapt the coax to the MCX input on the dongle.

This will give up to 200nm / 350km range

Then think about something better.

For people in high rises there are special considerations...

They will get the benefit from the height, but are disadvantaged by having to work behind glass (with whatever coatings it has to reflect various wavelengths for heat retention) and a restricted view (unless they can see out of all sides of the building)

[trigger]

I said this before, and Ill say it again...

You are using a VERY large hammer to crack the nut !
I'd bet that if you took a straw poll, 99% of the forum users couldn't care less about the "rocket science"... they only want to knock up a working antenna.
Matching feeder to antenna at these frequencies requires an accuracy few could attain, so is basically a waste of effort - without the correct test equipment totally pointless.

When I first got my TV dongle, I made a coco out of spare satellite cable... no idea of specs whatsoever ! 11cm a section.... fed with an old tv coax feed into the shack (again, zero maths)

Compared to a commercially built active scanner antenna (same feeder) the coco provides more than 10dB gain over the scanner antenna.

So YES you can lash up a good antenna with nothing more than a tape measure.

But there is the challenge of getting something to work better. Pushing the boundaries. It's what humans are made for.