Optimize the Whip Antenna Which Is Supplied With DVB-T Dongle

[abcd567]

STARTER
When DVB-T Dongle is purchased, a whip antenna is supplied with it. Every beginner uses this whip in his starter setup.

Improve the performance of this whip antenna by three simple steps given below:

(1) Whip is too long for 1090 MHz. Cut it to 67mm.
(2) Magnetic base provides an insufficient ground-plane. Enlarge ground-plane by placing the whip over a Metallic Can/Box/Plate. An added advantage of placing whip over an iron/steel can is that whip's magnetic base will cling to the can and keep the whip stable & upright.
(3) Signal is weak indoors. Place whip near a window or in attic to get maximum signal.

NEXT STEP
Once your setup is up and running, you will be happy, but soon after you will want to increase your range. You can then try some easy and sure antenna, like Spider & Cantena. Go to the thread linked below for "how to make" instructions for Cantenna & Spider

3 EASY ANTENNAS FOR BEGINNERS


VENTURING BEYOND......
Next step may be to venture through alluring coaxial antennas (coaxial cillinear, popularly known as coco, franklin, wire collinear with coils). These antennas are very alluring, as these are easy to make, and all postings on internet & youtube claim high gain excellent antenna. The problem shows up when these antennas are put to service, and give poor perfamance. Collinears are very hard to get right, and except few lucky ones, most DIY collinear makers end up with a poor performing antenna.


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[Lufthansa]

Hello,

i have several questions regarding the antenna whitch is supplied with the DVB-T Dongle and the optimization of it:

1. In your drawing you say that the antenna should be cut to the lenght of 67mm. I calculated that λ/4 for 1090Mhz is 68.8mm. Why is there a difference? (This is just a theoretical question. I know that the difference between 67mm and 68.8mm is very small and that in most cases the antenna will cut with a deviation of +/-3mm ).

2. I asked myself if the cable of the antenna act as an antenne itself. Is that right or is the cable shielded (like a coaxial cable)?

Kind regards
LH

[rueckwaertsflieger]

...should be cut to the lenght of 67mm. I calculated that λ/4 for 1090Mhz is 68.8mm. Why is there a difference?...

Because before you have to take care about 3mm of antenna length you better should ask from where the λ/4 should be measured. As you might have noticed, the length from the can to the antenna tip is even more than λ/4. If you measure this length, things are different.

I asked myself if the cable of the antenna act as an antenne itself. Is that right or is the cable shielded (like a coaxial cable)?

The cable is shielded. However, it might partly act like an antenna, because this type of antenna in no way will match the cable characteristics precisely, causing imperfect conditions for a cable to act as a perfect one. Just in case, you do not know better than abcd567, go with the recommended antenna length. The metal ground plane will have dramatically impact on reception compared to 3 mm of antenna length.

[Lufthansa]

Because before you have to take care about 3mm of antenna length you better should ask from where the λ/4 should be measured. As you might have noticed, the length from the can to the antenna tip is even more than λ/4. If you measure this length, things are different.

Yes, that's also a very good point. Where does the measurement starts? At the point where you screw the tip to the base of the antenna, or from the verry bottom of the base of the antenna to the point where you cut,.....

[F1Rocket]

The newer Nooelec antenna are tubular now. Besides the question of where to cut from, how about the hollow opening, does it affect anything? Thanks for sharing all of this knowledge, sure helps to get one up to speed.

[Kpin]

Yes, that's also a very good point. Where does the measurement starts? At the point where you screw the tip to the base of the antenna, or from the verry bottom of the base of the antenna to the point where you cut,.....

I would say the length should be measured from the end of the antenna cable - that is from where the core is free of the shield. And that is basically where the antenna cable enter the base.

Another question: The bottom of the base is covered. Should this not be removed so that the small magnet base gets in full contact with the can/groundplane

[abcd567]

I would say the length should be measured from the end of the antenna cable - that is from where the core is free of the shield. And that is basically where the antenna cable enter the base.

Another question: The bottom of the base is covered. Should this not be removed so that the small magnet base gets in full contact with the can/groundplane

Your both points are worth considering.

(1) Antenna length: length inside the base is a variable factor, depending on the manufacturer/design & worker who assembled the antenna (how much cable he left without braid). In the antenna I have, the total VERTICAL distance from the point where whip is screwed to base, up to the cable entry point is about 25 mm. If it is assumed that antenna length starts from cable entry point, then the whip should be chopped another 25mm i.e. 67-25=42mm. Since somewhat longer antenna is better than somewhat shorter antenna, I wont chop off entire 25mm. In fact couple of months ago I did the exercise below:

Starting at 67mm whip length (as in the photo n my first post), I observed the maximum range this antenna brings in for few days. I then chopped it off by 5mm, again observed for a day, found improvement. I did a 2nd chop of 5mm, and observed for a day, found improvement. I then did a 3rd chop of 5mm, and observed for a day, found performance slightly decreased, so I stopped and did not chop any further. For my antenna 10mm chop seemed good, i.e. whip length 57mm instead of 67mm.

Since your manufacturer may have a slightly different arrangement inside the base, you have also to repeat the stepped chopping exercise I have done to find best length for your whip.

(2) Touching metallic base of antenna with the metallic can or plate is worth trying, and may improve performance. I have never tried it. If you peel off the circular plastic tape from bottom of antenna, bare metal will be exposed. Placing it on metal can or plate will make direct electrical contact. This is easy and worth trying.

[abcd567]

FURTHER IMPROVEMENT
Recently I opened the base of the magmount antenna (images 1,2 & 3). I noted that there is about 15 mm of vertical metal part between screwable whip and feed coax.

This hidden vertical metal is part of antenna, and hence antenna length should be measured from the bottom of this part where coax is soldered to it. Since this part is 15 mm long in my antenna, my magmount's length was actually 67 mm + 15 mm = 82 mm. I therefore chopped off another 15 mm from the whip, making the length of removeable part 52 mm instead of 67 mm. See image 4. I noticed about 5% to 10% improvement.


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Images 1, 2, & 3
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Image 4


Things to be tried:
1. Remove black plastic circular adhessive cover from bottom of antenna, and check electrical continuity between exposed metal base plate & the outer metallic part of MCX connector at other end of antenna's coax.

2. Place exposed base on a metallic can and check if direct contact gives any improvement.

[Adriaan25]

As a starter with this setup I've entered the "okay cool it works, now how can I increase range"-phase. So I'm following with increasing interest!
I'd like to get experimenting with making cuts and extending the ground plane as well, which I will in a week's time ("unfortunately" got a holiday first).

Thanks for your information so far abcd567 & others.

[RIN67630]

STARTER
(1) Whip is too long for 1090 MHz. Cut it to 67mm.

The whip is made of an astonishly hardened steel. Use very strong plyers to cut it. I have almost ruined my electronic cutting plyer upon trying to shorten it!

[RIN67630]

Your both points are worth considering.

(1) Antenna length: length inside the base is a variable factor, depending on the manufacturer/design & worker who assembled the antenna (how much cable he left without braid). In the antenna I have, the total VERTICAL distance from the point where whip is screwed to base, up to the cable entry point is about 25 mm. If it is assumed that antenna length starts from cable entry point, then the whip should be chopped another 25mm i.e. 67-25=42mm. Since somewhat longer antenna is better than somewhat shorter antenna, I wont chop off entire 25mm. In fact couple of months ago I did the exercise below:

Starting at 67mm whip length (as in the photo n my first post), I observed the maximum range this antenna brings in for few days. I then chopped it off by 5mm, again observed for a day, found improvement. I did a 2nd chop of 5mm, and observed for a day, found improvement. I then did a 3rd chop of 5mm, and observed for a day, found performance slightly decreased, so I stopped and did not chop any further. For my antenna 10mm chop seemed good, i.e. whip length 57mm instead of 67mm.

Since your manufacturer may have a slightly different arrangement inside the base, you have also to repeat the stepped chopping exercise I have done to find best length for your whip.

(2) Touching metallic base of antenna with the metallic can or plate is worth trying, and may improve performance. I have never tried it. If you peel off the circular plastic tape from bottom of antenna, bare metal will be exposed. Placing it on metal can or plate will make direct electrical contact. This is easy and worth trying.

Would't it be much easier to trash completely the whip antenna and build a dipole with 65 mm of the core wire at one side and 6,5mm of the shield at the opposite?

[RIN67630]

By the way: did someone measure the velocity factor of the supplied coax cable?
I can imagine, that a λ/4 T stub could well improve the overall sensitivity by short-circuiting the lower-band signals and avoid useless load on the RTL chip.
Do you agree?

[rueckwaertsflieger]

Sleeve antenna

Would't it be much easier to trash completely the whip antenna and build a dipole with 6,5 mm of the core wire at one side and 6,5mm of the shield at the opposite?

1 In case you are willing to increase your targeted antenna segment length 10 fold, you might have a chance. The length of both the active elments should be less than 69mm.
2 The design you are targeting probably is just one kind of a sleeve antenna. Turning back the cable screen around the feeding coax with coating after stripping the cable coating for a magnitude of less than 65mm - around 45? depending on cable type - and using around 65mm inner conductor as whip. Theoretically you might get rather good results with this antenna. In practice the characteristics are defintitely sensitive to tiny changes in geometry. Further, you have to optimize length of shield folded back and whip experimentally. The screen has to be shorter than wavelentgh in free air by a factor near the velocity factor of the cable and so on.
3 In case you will try this design, use very thick cable and a rigid sleeve. This will somehow make your design more robust. Probably you could choose Ecoflex 15 plus copper tubing 15mm for the sleeve, connected to the shielding.
4 In case, you won't have success, go back to the cantenna design, turning out to be a somewhat sleeved antenna with dimensions such big, gaining much more robustness against mechanical variation.

However, a not optimized sleeve antenna definitely will outperform the funny whip antenna provided with the electronics as long as you go close to half wavelength with both the active elements - in other words: do not try 6.5mm but less than 69.

Good luck

[RIN67630]

Oops, you ar right: I meant 65 mm or 6,5cm. It was a typo.

I did not mean turning the sleeve back, but in a T-form.

That is what i've made with regular Sattelite antenna cable, since I needed a longer cable:
Beast4.jpg

The wire and sleeve are just placed under the duct tape. This is a dirty construction placed inside a wardrobe under the roof. I will make a better antenna later, when the wheather permits.
You may use other non conductive supports as e.g. gardening rods to support the construction outdoors.
Using the original cable without the losses in the intermediate connectors shoud be good as well.

[abcd567]

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@RIN67630:
@rueckwaertsflieger:


(1) My Tiny Tot, Kleinkind, Dipole: 6,8cm + 6,8cm

(2) GROUNDPLANE ANTENNA MADE OF COAX ONLY - Without SO239 or N-Conector

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