RTK Receive Only Antenna, Unity Gain or higher Gain antenna?

[FlightSim]

Hi, I'm looking at a system using RTK Corrections. I wanted to check if it mattered if I used just a Unity Gain Antenna for the Mobile Vehicle that a Receive only antenna is mounted on. The Corrections are sent from a base which I know is important for choice of antenna depending on the range or enviornemnt the transmitting signal is progagate over but wanted to check if it mattered for the vehicle antenna end as this is going to be receive only. Does it make any differnece what antenna gain is choosen for reception performance.

thanks

Paul

 

[VEBill]

RTK = Real Time Kinematic(s)

If you use a high gain antenna, then you have to aim it in the correct direction (otherwise you'll be relying on the lower-than-low gain portion of the pattern). That's a bit complicated which is why most vehicles use low gain, omnidirectional antennas that don't require complicated antenna aiming systems.

Regarding the receive only, the theorem is called Reciprocity.

 

[VEBill]

PS: There are "high gain" omnidirectional antennas, they have a pattern with higher gain towards the horizon and less wasted up and down. If your vehicle stays level (etc.) then they might be useful.

 

[FlightSim]

So its best just to go with the omniderectional unity gain antenna for the vehicle based antenna at compromise that the sensitivty will be reduced due to having a lower gain, also that the antenna aerial is shorter for the lower gain? The vehicle will be used in all kinds of terrain not just on level planes.

The antenna tracking is an intresting area but for the purpose of RTK Corrections for mobile mapping want to stay away from it. Looking at it though for airborne application for video thou.

thanks Paul

 

[VEBill]

Yes, vehicles that can manoeuvre require low gain antenna systems that can pick-up signals from any relative direction.

If it's a land vehicle it opens up the option of omnidirectional higher gain antennas, but don't push the high gain too far (roughly above 10dBi) if the vehicle will be tilted in rugged terrain.

For investigative projects (where there are people available), one could use a very high gain antenna at the base station and manually aim it towards the vehicle. An example you'll have seen is animal tracking collars where the transmitting collar antenna is very low gain (because you can't predict the orientation of the animal), but the biologist holds a relatively high gain Yagi antenna and manually aims it.

 

[FlightSim]

You mentioned not going roughly above 10dbi. I've read up a little on db, dbm and dbi. dbi is mentioned as a good one to use for antenna comparison but db seems to be quoted for the antenna's I'm interested in such as the pctel one

http://www.ascommunications.co.uk/radio-data-systems/adl-vantage-antenn/

Is this 5 db that's actually quoted in fact inferring dbi and not just db?

thanks Paul

 

[VEBill]

"5dB" just means that something is +5dB more (linear factor of about 'times 3.16') than something else. If the something else (the reference) is undefined, then it might be anything.

In the case of antenna gain, it's either dBd (referenced against a dipole), or dBi (a hypothetical isotropic antenna). These are the most common baselines for antenna gain. The dBi number would be 2.15 dB higher than the dBd; in other words, a theoretical dipole has a gain of about +2.15 dBi. Most manufacturers and vendors will use the dBi because it gives a larger number, and it has a better theoretical basis.

The ADL/Pacific Crest line of antennas are probably dBi, but they've managed to make it unclear. They have "unity gain" dipoles, implying either 0 dBd (unity) ...or perhaps 0 dBi (unity) (the directionality of a dipole, but a bit lossy?).

In spite of this confusion, it looks like a nice line of antennas. An elevated feed (vertical) dipole is a good thing. +5dB(probably i?) is not enough to be too directional.

 

[FlightSim]

Yes, checked and the db they are quoting is actually dbi.

I'm looking at using Omni-Directional Collinear Antenna (HD 6db Gain antenna, with fly lead (overall length 3.05 metres) ‘N’ type connector). Using this with a ADL Vantage Pro (max 35W). On the license I have, I can transmit up to 50W with 12.5 Khz bandwidth. This antenna is suitable for the Radio unit, but using the radio at 35W with this 6db antenna is 35W the effective propagated power (ERP). Want to make sure I'm not exceeding the max of 50W ERP.

thanks Paul

 

[VEBill]

using the radio at 35W with this 6db antenna is 35W the effective propagated power (ERP).

I'm more familiar with EIRP, but ERP is explained here, specifically as defined in the USA, and that explanation seems pretty clear. In this example, 35 watts + (6dBi antenna - 2.15 dBi/dBd ratio) = 35 watts + 3.85 dB = 35 x 2.43 = about 85 watts ERP. Minus a bit for cable losses (not much). So you're a bit high.

If you're in the USA, you better change the design to stay within the limit of your license. The FCC can and will hand out Administrative Penalties of thousands of dollars.

You're allowed +3.7 dBi net antenna gain, or +1.54dBd. This is the ratio of 50 watts to 35 watts.

This assumes that your RF output actually *is* 35 watts.

Safest approach is to select a "unity gain" antenna. Keeps you well away from any violations.

Please double check my math, as it's early and I've not yet had my coffee.

 

[VEBill]

Oh, wait a second.

Your mobile is RECEIVE ONLY ?

Then you're allowed as much antenna gain as you want.

 

[FlightSim]

Actually you were right was the transmit end I was talking about. Collinear Antenna I was talking about was for the base station end. Also its for use in UK Scotland. The ADL Vantage Pro power level is settable so can reduce this to keep within limits. Able to use either 12.5KHz to 50W ERP or 25KHz bandwidth to 100W. Max output from the Radio 35W. Is there any website you'd recommend to look at for some of the theory so I can read up further? Thanks Paul

 

[VEBill]

For looking up individual subjects or concepts, Wikipedia is usually quite efficient. Of course you need to keep your skeptical thinking switched on in case you run across a badly written or vandalized subject. But as Wikipedia is constantly monitored and peer-reviewed, it's a much better reference than many people believe. There are usually links to outside sources listed at the bottom, so one can further explore.

Essential topics include: dB concepts (e.g. 3.01db and 10dB), antenna gain vs directivity, antenna gain patterns vs your coverage requirements, cable loss, path-loss (Google the number "32.45" and you'll find it), noise figure for the LNA, Shannon–Hartley theorem & bandwidth impact on noise, and many more.