Easily Improve & Optimize the Magmount Whip Antenna Supplied With DVB-T Dongle

[ab cd]

The magmount whip antenna supplied with DVB-T dongle is too long for 1090 Mhz. It also has a tiny base, which is ok as magmount, but too small as groundplane.

I have been using the DVB-T's magmount whip antenna after trimming its removeable part to 67mm length (i.e. 1/4 wavelength at 1090 Mhz), and placing it over a metallic can to enlarge ground plane as shown in the image 1. Above noted two steps improved the performance of whip by 30%.

Image 1 of 5

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FURTHER IMPROVEMENT
Recently I opened the base of the magmount antenna (images 2, 3 & 4). 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 5. I noticed about 5% to 10% improvement.


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Images 2, 3 & 4

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Image 5

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.

 

[Lee Armstrong]

Great advice!

 

[ab cd]

The 1090 Mhz frequency is used for Extended Squitter, and is called "1090 ES". This system is now in use all over the world.

In USA and Canada, an additional system called "978 UAT" (Universal Access Transponder) is in use. This system operates at 978 MHz, and is restricted for use by aircrafts flying 20,000 feet and below.

Since there is not a big difference in these two frequencies, there is not a big difference between the lengths of the 1090 Mhz & 978 Mhz whips.
The quarter wave whip for 1090 Mhz will also work for 978 Mhz and vice versa. However if someone is interested in using exact length for each frequency, following are the calculations.

1090 Mhz
Wavelength = 300,000/1090 = 275 mm
1/4 wavelength = 69 mm
Whip length = 1/4 wavelength - reduction for end effect = 69 mm - 2 mm = 67 mm

978Mhz
Wavelength = 300,000/978 = 307 mm
1/4 wavelength = 77 mm
Whip length = 1/4 wavelength - reduction for end effect = 77 mm - 3 mm = 74 mm

 

[Malpractice]

I just wanted to feed back on this tip.

I saw a massive increase in position reports (est >50%) and a small increase in range when I shortened the antenna to ~67mm from the full height. I recommend that people try this, it definitely works!

However, I am sure the antenna theory is correct but in my practical experience when I reduced it to ~52mm my position reports dropped by approx 30% and the range dropped by 20-30nm with the antenna in the same location.

As I only have 1 receiver and I am well aware that traffic varies but I have not been able to improve on the range and position tally that I had when I had a the 67mm antenna. Maybe the sweet spot is somewhere in-between!

I can be confident as even when we had transatlantic traffic holding at >33k ft above us when Heathrow had the fog and emergency a few weeks ago it didn't beat a normal day of position reports. I see lots of gaps in dump1090 tracks i didnt have before as well.

I have now bought a 'proper' antenna so the old whip will be retired soon...

 

[ab cd]

@Malpractice:
Thanks for notifying your experience in this forum.

Antennas are somewhat tricky. Often these defy theory, and sometimes give conflicting results with different users. I got improvement by cutting additional 15mm. Let us wait for someone else to post his experiance. What majority experiences is to be taken as fact.

I will try to repeat trimming experiment on another new mag mont whip to see what happens with new one.

Other forum members, who have done the trimming, are requested to post their experience.

 

[ab cd]

@Malpractice:
Since antennas supplied with dvb-t by different sellers/manufacturers may be slightly different, it is possible that in some antennas the inner metallic part is shorter or longer than the 15mm I found for my antenna. Have you removed the base plate and tried to measure the length of metal part inside the base?

 

[Malpractice]

I think my antenna cable or internals may have been at fault. Every time I moved it, even higher, the signal seemed to get worse and worse. It also the antenna may have started to corrode as I live 1 mile from the sea. I have binned it now as my SCO-1090-MCX antenna has arrived.