RG174 has attenuation of 1dB/m
LMR195 has attenuation of 0.36/m
V-Stub’s 3m RG174 has same attenuation as 9m (30ft) LMR195.
FA antenna’s 2m RG174 has same attenuation as 6m (20 ft) LMR195
The 9m (30 ft)LMR195 is the cable run of an antenna mounted on 20ft mast or roof of 2 storied house, a very normal scenario attenuation wise. You can take my setup as V-stub connected to dongle with 9m LMR195.
This is one of the “7dBi” Magmount antenna with 3m RG174 on ebay, $3.59+Free Shipping.
I have purchased a similar antenna some time ago, and have recently replaced its whip by my V-Stub whip.
Wow what a fantastic antenna for just $3.59+Free Shipping.
Description:
Product specification:
Frequency range (MHz),:800MHz, 900MHz, 1200MHz, 1500MHz, 1800MHz, 1900MHz
Gain (dBi):7
S.W.R: < 1.5
Input impedance (Ohm): 50
Power capacity (W): 100
Antenna height (mm):280 Cable length (m):3 Cable type: RG174
Connector type: SMA male
Yep!
Ignoring the fact that this is a rubbish antenna that can’t possibly live up to its claimed specs, have you seen anything to indicate the coax is actually RG174?
The Cheap’n’Nasty mag antennas I’ve got use cable more closely resembling audio coax than a respectable RF grade.
If V-Stub’s signal after passing through 3m of an audio grade coax gives the aircraft/message figures as in my two previous posts, and max range of 250 nm, then this goes to prove that V-Stub is a really good DIY antenna
Granted you are receiving data, but I’d be more concerned about the cable being able to handle the higher frequencies that antenna advertises, given most companies rate the max frequency at 1000MHz and 1090 and other freqs are above that.
I know there are some high-end companies that make a higher grade RG174 that can go higher, but I doubt anyone is using those, and the majority are rated to cap at 1000MHz.
But hey if it works, I guess all good.
Personally I would go with LMR-240 or better, but that is just me
The Magmount antenna supplied with the DVB-T Stick or purchased alone is very commonly used by those who cannot install an outdoor antenna or those who just started this hobby by buying a generic DVB-T which is paired with a magmount antenna.
The magmount antennas generally have only 1 m cable, so although the cable is of bad quality, due to it’s short length it’s total loss is around 1 dB. Even if it’s integral coax is replaced by a low loss coax of 0.3 dB/m, it will result in maximum benefit of of 0.6 dB.
Normally users cannot / would not change magmount’s integral cable. However they often improve it’s performance by (1) placing it over a metallic surface and (2) cutting the whip to 1/4 wavelength.
I have gone one step further, and instead of cutting the whip, replaced it by a DIY V-Stub wire whip, which changed antenna gain from less than 2 dBi to 4 dBi, and this has brought a substantial improvement.
How the Magmount’s ground plane is enlarged by placing it over a metallic plate when there is no metal to metal connection between coax braid and metallic plate?
The braid of coax is connected to the 1 inch (25mm) dia base plate. A small magnet of about 10mm dia is glued to the base plate. Please see photo below. The base plate and magnet is covered by a black circular adhesive film, hiding them from view.
The base plate is the ground plane, and magnet keeps the antenna attached to a iron plate (like roof of a car or a food can), keeping it upright and firmly held in position
The electrical connection between antenna’s base plate and the food can or car roof is not direct, as there is a black circular insulating tape over base plate. It is through the capacitance formed between base plate and food can which provides the electrical connection
The antenna’s metallic base plate disc is hidden behind the disc shaped plastic tape. The disc is about 1 inch (25mm) in diameter. With a thin plastic tape (0.5mm thick, dielectric constant 2.2) between the can & base plate of stock whip, the capacitor formed is of the order of 20 pF
At 1090 mhz , the 20 pF capacitor’s reactance is low enough ( 7.3 ohms ) to provide a reasonably good connection.
Below is the preemptive answer to the rough crowd’s expected question “how do you know that capacitance between magmount’s base plate and metal sheet is 20 pF?”
Capacitance in Farad = 𝜅 x ε0 x area of plate m2 / separation of plates m
ε0 = permittivity of space = 8.854 x 10-12 Farad/meter
𝜅 = relative permitivity of dielectric material between plates
Magmount’s base-plate’s diameter is 1 inch / 25mm
Area of plate = π x r2 = π x 12.52 mm2 = 491 mm2 = 491 x 10-6 m2
𝜅 = 2.2 (estimated dielectric constant of circular black adhessive platic film covering base plate)
Separation of plates = 0.5 mm = 0.5 x 10-3 m (thickness of black adhessive film)
Capacitance = 2.2 x 8.854 x 10-12 x 491 x 10-6 / 0.5 x 10-3 Farad
= 19128 x 10-15 Farad = 19.128 x 10-12 Farad = 19.128 pF
Yes, you are right, the whole disc is not 0.5 mm seperation, but you are also wrong that it is only the rim, as you overlooked the area of the magnet (the circular tablet in the middle of disc).
It will need a more precise calculation, but even if the area, and hence capacitance works out to be 1/3rd (i.e.20/3 = 7 pF), the capacitive reactance will be 3 times i.e. 3 x 7.3 ohm = 21.9 ohms.
I was only pointing out the most obvious flaw, not all of them!
The magnets in mine are significantly recessed.
Ignoring the points above, knowing the capacitance doesn’t really help unless you are chasing a target.
One thing you could try is removing the sticker and placing it on bare metal (say the inside of your Danish cookie lid). It might make electrical connection without relying on capacitive coupling.
It’s not that the cable cannot pass frequencies above some limit stated in the spec sheet, but that specs are not guaranteed (maybe not even tested). The impedance may not be 50 ohm, or the VF or capacitance/ft may be different.
I’m sure that 174 will pass 10GHz signals, just not very well, and with a lot of loss.
Since the V whip has substantially improve the performance of stock whip even without metallic connection to window sill, the need to further improve the ground plane was not felt, but I will still try it.
Actually I wanted to keep things simple and easy for a startup hobbyist. Placing magmount on metal plate without any modification is the easiest thing to do, and most new hobbyist will happily do it. If they are told to remove adhesive tape and stuck some foil underneath, many wont do it.
give them “basic” design and “advanced” design. since they can do the basic design and then “upgrade” to the advanced version i think most will try the advanced.
Improvement 1 (Easy) - Improve ground plane by placing over a metal sheet. (DONE in 2015 in another thread)
Improvement 2 (Easy) - Chop whip to 1/4 wavelength, reducing SWR to 1.5. This however does not improve gain, which still remains less than 2 dBi. (DONE in 2015 in another thread)
Improvement 3 (Advance) - Change straight whip by V-stub whip. This reduces SWR to 1.2 & Gain to 4 dBi. (DONE in 2020 in this thread)
Improvement 4 (Advance) - Remove plastic film from bottom of magmount, and stuff some aluminum foil to make a direct connection betwern metallic sheet and base plate. (To be done)
Improvement 5 (Advance) - Change magmount’s integral 1m cable by a better 1m cable and terminate new cable in a connector for dongle. Not practical as physically a larger diameter cable cannot be easily inserted in small magmount base. Also the 0.6 dB improvement is not worth the effort. (NOT to be done)
Magmount antenna with 3m RG174 on ebay, $3.59+Free Shipping.
