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Oct 13, 2019, 09:33 AM
Registered User
Flynn203's Avatar
The 5.8 version is a $100 cheaper, so affordable for me, if it will make a worthwhile difference.

I wonder whether a 23 dBi antenna will give a better image at say, 15km than the Gatling at 15.5dBic.? Or is it simply a matter that the higher gain antenna will be capable of receiving a better signal much further out, but both will be equally good at medium range.

That leads to my next question - TrueRC, for example, quantify antenna gain in dBic while the discussion here uses dBi. Googling this, I get:

'Circular polarization antenna gain is measured in dBiC
The gain of an antenna with circular polarization depends on the axial ratio (AR).
It can be seen that an antenna with an axial ratio of 0.0 dB its gain is 3 dB higher than the gain of a linearly polarized antenna. '

and

'when you talk about an antenna gain, you should specify your theoretical antenna reference if it's hypothetical lossless isotropic antenna (then it's dBi ) . An alternative definition compares the antenna to the power received by a lossless half-wave dipole antenna, in which case the units are written as dBd. Since a lossless dipole antenna has a gain of 2.15 dBi, the relation between these units is:
gain in dBd = gain in dBi - 2.15 dB
Generally dB is the 10 log(power expressed in W)
Generally dBm is the 10log (power expressed in mW)
you are right for HFSS (That I work with) the implicite gain is a dBi (called also null reference) . BUT for the polarization gain, it's dB because you can't talk about dBi nor dBd because you don't need a reference. the polarization gain is a ratio of radiatting between a max and min radiation (in two perpendicular directions ).'
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Oct 14, 2019, 04:42 AM
FPV Enthusiast
Closus's Avatar
Quote:
Originally Posted by Flynn203
The 5.8 version is a $100 cheaper, so affordable for me, if it will make a worthwhile difference.
I wonder whether a 23 dBi antenna will give a better image at say, 15km than the Gatling at 15.5dBic.? Or is it simply a matter that the higher gain antenna will be capable of receiving a better signal much further out, but both will be equally good at medium range.
Usually yes, with a higher gain antenna (assuming it's aimed correctly), you do get better image at ranges where your video signal won't be perfect anymore. For <10km, it won't matter, but further out you'll notice the difference. While the lower gain antenna will still be very usable (and much easier to aim), a perfectly aimed higher gain antenna will outperform it in my experience.
Also, on 5.8GHz your video is impacted quite a bit by outside factors such as weather, humidity etc., so you might not always notice the difference between higher and lower gain antennas equally much.

Quote:
Originally Posted by Flynn203
That leads to my next question - TrueRC, for example, quantify antenna gain in dBic while the discussion here uses dBi. Googling this, I get:
'Circular polarization antenna gain is measured in dBiC
I think it's usually pretty comparable as most antennas used for FPV are circular polarized. However once you start going for very long range, lots of people (me included) start using linear antennas, as it's easier to find linear polarized patch antennas with very high gains (23dBi and up). Both are viable solutions however, and high gain linear antennas come with the disadvantage that for example when banking your plane 30 degrees while 50km away, you will almost completely lose signal as the VTX antenna is no longer vertical - so very flat turns are necessary at long ranges.

Again, all this is just based on my experience and YMMV I'm not exactly an expert on antennas, but have found that it's pretty easy to go far on 5.8G with the right gear.
Oct 14, 2019, 04:47 AM
Registered User
Flynn203's Avatar
Thanks for that. I have 1.3 gear in a box, but have never bothered to use it. I'll maybe give the Gatling a try.
Oct 14, 2019, 05:13 AM
Registered User
Those figures are pure theoretical and for fixed installations.
Weather conditions and antenna position changes can affect the whole link budget even by 15 dB, which is the default margin for most budget link calculators.
For example 3 dB is a sure loss only by banking the model by 30 degrees...

Scotland weather is not the most stable I know for trying records for link at distance...
Another factor... the country is so tiny... whatever being your location inside the country, you have a maximum of ground 80 km, in a direction. Maximum ! So there is at least a sea shore at less than 80 km around you.
I am sure Flynn knows this, this info is for general public.
Oct 14, 2019, 05:24 AM
Registered User
Flynn203's Avatar
Ha - you are correct, moist air is not so good, and we have a lot of that. A mate of mate flies out from the coast to local islands and seems to like 2.4 video.
Oct 14, 2019, 06:35 AM
FPV Enthusiast
Closus's Avatar
Similar problem here in Switzerland, no matter where you go, you'll certainly have to go over mountains to go a decent distance!
Oct 14, 2019, 07:31 AM
Registered User
Thread OP
I would not say that environmental factors impact that much the 5.8 GHz. Yes, it is more affected by rain and moisture than lower frequencies but:

on the theoretical side: there is data that light rain (2.5 mm/hr) absorbs 0.005 dB/km and 12 mm/hr 0.002. That means about 0.25 and 1 dB/50 km... you will never notice that. But with a 12mm/hr rain the loss of video quality will be your least worry... The lens will completely clog in a couple of seconds... and it would be very interesting to know how long the electronics will hold. I can't imagine anyone doing long range flights even with 2.5 mm rain... Now, if you have an occasional strong rainwhower, it would be localized and again would not affect the image.

on practical side: several times I flew through low cloud layers (200-300 meters thick); when flying long range, I had altitude/distance ratio of about 3 %. That means about 10 km of clouds between the antennas. I never notices any difference...

For the antennas. There are obviously good and bad antennas, but there are no "magical" antennas, i.e. if you pay double or more, it does not mean at all that you would get much better quality (and range). I think that any reasonable professional manufacturer has good antennas, like L-Com, and I never bothered purchasing special FPV antennas. Also, with dish antennas there is much less which can go wrong, so for higher gain antennas I definitively would prefer dish - the design of efficient patch antennas is quite complicated and is a science for itself. The dish is much simpler - you have a reflector which kind of should focus the beam in one point and a dipole receiver antenna. Unless dish is out of shape or out of focus, it would always work.

If I were on a tight budget, I never would buy expensive patch antenna. A bigger, relatively "good" antenna would always outperform smaller, "best quality" antenna. And a dish antenna is even better... One warning: stay away from chinese antennas, they may indeed be empty boxes or have this disaster "design" which appeared int his thread a couple of pages ago. I would definitivwly buy local from a reputable antenna provider, but not necesarily for FPV use.
Last edited by chileflora; Oct 14, 2019 at 07:42 AM.
Oct 14, 2019, 09:21 PM
'Extreme Fabricator'
Here's another one for the mix, I have found there to be a big difference in range between day and night also. I have a night flying wing that easily gets 5km at night with just 200mW, during the day 3km is pushing it. I also tune my home made 5.8 antennas at night as the signal transmitter I use (1mw @ 250m) gives a cleaner response on my spectrum analyser when its dark, so my guess is some portion of solar radiation is also noise for the 5.8 spectrum.
Last edited by alicecooper; Oct 15, 2019 at 08:16 AM.
Oct 15, 2019, 06:17 AM
Registered User
Interesting observation there.
Early radar experiments in the 1930s were plagued by a diurnal hiss which was initially thought to be related to solar radiation but later it was found to originate from the center of our galaxy. There is something called the 21cm Hydrogen line which produces radiation from neutral Hydrogen on a frequency of 1.4204GHz which curiously is 1/4 of the frequency of Band E, Ch 2
I would doubt there would be harmonics associated with this natural phenomenon and I would hope the receiver would have enough front end filtering to reject this frequency but you don't need a lot of non-linearity to generate harmonics which can then be amplified and detected.

Cheers,
David
Oct 15, 2019, 08:40 AM
'Extreme Fabricator'
Quote:
Originally Posted by KiwiDavid
Interesting observation there.
Early radar experiments in the 1930s were plagued by a diurnal hiss which was initially thought to be related to solar radiation but later it was found to originate from the center of our galaxy. There is something called the 21cm Hydrogen line which produces radiation from neutral Hydrogen on a frequency of 1.4204GHz which curiously is 1/4 of the frequency of Band E, Ch 2
I would doubt there would be harmonics associated with this natural phenomenon and I would hope the receiver would have enough front end filtering to reject this frequency but you don't need a lot of non-linearity to generate harmonics which can then be amplified and detected.

Cheers,
David
I need to try 5682 to compare noise levels... That is interesting, I knew there was noise everywhere, but that specific frequency is worth a look.
Where I live (in the far southern hemisphere) I also get some amazing aurora australis displays, with sweeping waves of light playing tricks on the eye, nearly every night there is a distinct glow to the south which shows there are a lot of charged particles up there, down here... I wonder if that's what limits my daytime range with 5.8gHz, compared to some my 10km limit for 800mW seems paltry. I have no long range issues with 1.3gHz, but 5.8 with everything needed to go far it just doesn't cut it. I still use 5.8 for most builds, as I don't venture really long distances anymore, but seeing others hit 30+km is intriguing. At the other end of the power scale, I can regularly get 1500m with under 25mW on several ultra tiny (~50gram) FPV planes. Maybe at night I could get 30km with a watt...
Oct 16, 2019, 05:30 AM
Registered User
Hi,
I think it's a long shot that they are related but you never know. If the video receiver has any amplification prior to filtering the band required then there is a chance the non-linearity of that first stage can generate harmonics and the 4th harmonic would then go through the remaining amplifier stages and be detected. I assume you have ruled out any chance that the day/night difference could be temperature related? Some of the circuitry for the smaller vtxs appears to be baby video minder chips running at their maximum rated voltage to get more power. Often too the linear regulators are running so hot they would almost slide off the board if held vertical. (well very hot anyway)
1500m on 25mW is impressive. That has to be either very good receiver sensitivity or a gain antenna or both.

Cheers,
David
Oct 16, 2019, 06:34 AM
'Extreme Fabricator'
I have wondered about other possibilities causing the difference, temperatures do plummet at night, usually below dew point, but not so much in summer when I usually fly. I have had many different receivers over the years, from R-600's with around -95dBm, two FR632's, IRC Duo 5800 (still the best of the bunch), and a few different diversity modules for direct goggle use, and one (Furious TrueD) which also became a grounstation antenna...
I have a few building and tuning techniques for 5.8 antenna's that get's them performing substantially better than my benchmark Circular Wireless 8 Turn helical's, with lower frequency antenna's being much easier to make and tune with a VNA. I know my range would be lower with off the shelf stuff, but spending eight hours on an antenna is half the fun of the hobby... Still, the sub 10km I get still fit's perfectly for 90% of my builds, and maybe if I pushed through fresnel zones it would prove a different story, but terrain doesn't work in my favour there.
You're right with the VTX's, they are generally very poor for efficiency, using a simple class A output stage with generally just a stressed out Gallium Nitride transistor running hard bias and low gain, the combined driver and output stages usually barely passing 25% efficiency and actually utilising their thermal instability to self limit the C/E current, along with poorly designed RF coupling and thermal management it's a wonder many last at all. Now most of my VTX's get some treatment to extend their life, and several brands have proven so bad, after post mortem analysis of failure I won't risk touching them again (Foxeer for one) .
As for my range with low power, I only get that running a tripod mast around 4.5m high and the Duo5800 RX(usually) with helical's or patch mixes up top with a pan tilt. Elevation of the receivers and antenna's is one of the best range boosters available.
Oct 16, 2019, 07:01 AM
Registered User
Thread OP
Do you think that IRC Duo is better than RC305 in terms of sensitivity? I have not tried this one...
Oct 16, 2019, 07:15 AM
Registered User
Thread OP
I have to disagree about height of the antenna. If you have clear line of sight, going higher will not help. Freshnell radius is about 1 m close to the antenna, so if your antenna is above this point, there will be no improvement going higher - at 100 meter, the freshnell zone is 2 m. That gives you a slope of 1 % (if your antenna is 1 m high). I often fly at 2-3% slope and never encountered signal problems unless there is vegetation blocking, and the lower part of my main antenna is just 1 m high...

Very good VTXs are from AKK. They were a real game changer for me - very consistent power output and it seems that they have active temperature control. For the last two years that the only brand I am using, and I think that they prevented me from cswithcing to a lower video frequency. Before, I was buy VTXs and same brand and model could have extreme variability in its power output.

I have flown on 25 mW SETTTING (not actually measured output) to about 20 km. barely flyable though...
Last edited by chileflora; Oct 16, 2019 at 07:36 AM.
Oct 16, 2019, 07:55 AM
Registered User
+1 agreeing about antenna height.

Quote:
Originally Posted by chileflora
Do you think that IRC Duo is better than RC305 in terms of sensitivity? I have not tried this one...
I doubt, their Airwave based stuff was always a step behind SkyRF.
Didn't seen yet a flight even half of your distances using iRC gear.


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