VHF Antenna

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Thread Starter

Iqbal

A VHF 3db gain antenna is installed on a 100 feet tower. The length of the coaxial feeder RG-213 is 250 feet. the output power of the radio set is 25 Watt. The radiating power was 1.5 to 2 watt measured before installation with same feeder legth. the communication range was extended fro 40 KM to 55 KM.
I want to extend the communication range more than 80 KMs. If I increase the power of both sets to 45 Watts each, than how much power will be lost in cable. It is not feasible to install a seperate tower near Communication Room.

Is there any formula to calculate the RF power loss in this regard.

Thanks
Iqbal
 
Junk the RG213 cable that is VERY LOSSY at that frequency. Replace it with Belden 9913U and buy good connectors. You will gain 3db worth of cable loss and have much more range. Remember you will gain receiver sensitivity also. Better yet go to 5/8 inch hardline. Yes it is more money but the results will be well worth. Antennas and feedline will more than make up for the effort.
 
Curt, this is the single biggest mistake one could make . Garbage in, garbage out. The point where one can make the largest gains with the least investment is the recevier input. A good antenna and the lowest loss cable is the best approach. 250ft of lossy cable can kill even the best receiver in the world. The gain is twofold, both more tx power and more rx sensitivity . The OP said something along the lines of measured power was 25W at the tx output, and 1.5W at the antenna . Almost all the tx power was lost in the feeder, and the same attenuation happens at the receiver end. That is simply way too large. If he doesn't need the range, buy a less powerful transmitter, a smaller gain antenna but don't screw your receiver input signal.

Matt Tudor , MSEE
http://www.gigahertzelectronics.com
 
M

Michael R. Batchelor

I'll second Curt's assessment. Increasing gain of the antenna is almost always the easiest and cheapest way to go. 3db gain is nothing. You can easily get 5db to 9db by spending a few hundred
dollars or less. (And if you can use a directional antenna you can get 20db or more with a well tuned Yagi, but this assumes you don't need 360 degree signal coverage.)

You can predict the signal loss from the chart here. It's dependent on frequency, and you didn't specify that part.

http://www.radiobooks.com/products/techinfo/coaxloss.htm

Note, if you have 1.5 watts ERP (effective radiated power) now, and you double the gain of the antenna, you'll double the ERP without changing the output of the transmitter. However, you'll need to check what your licensing authority says you can do. If it's a commercial installation you probably can't just change it
without permission. Same with the output power of the transmitter. At least that's how it works where I live.

Michael

--
Michael R. Batchelor - Industrial Informatics, Inc.
Contribute to society: http://www.distributed.net/ogr/
 
M

Michael R. Batchelor

Uhmmm... He said he was using RG213. That's hardly junky cable, but you're correct it isn't as good as hard-line. However, 250 ft of hard-line will cost thousands of dollars. And gain is gain is gain. 3db of gain in the transmitter (25 watts to almost 50 watts is 3db) is exactly the same as 3 db of gain in the receiver is exactly the same as 3db of gain in the passive pieces (the antenna and transmission line). The cheapest place to get 3db of gain is the antenna. In fact, if you change the antenna on *BOTH* ends you get even more gain in the overall system.

I assume from your comment your planning to lower his signal losses rather than increase his transmission system gain. But while you out there in the passive copper and aluminum pieces you're not adding noise to the system when you monkey with the gain or attenuation numbers. You add noise when you get into the electronics. And removing 3db of loss in the cable is exactly the same as adding 3db in the antenna.

A "good" antenna depends on the need, and a 3db gain $5K commercial installation has exactly the same ERP as a stacked 5/8 wave I can make in my garage out of old television antennas. And the commercial job will be even less effective compared to a 15 element Yagi I can build out of a few more old TV antennas. Of course, I won't guarantee mine against wind loading, and I'm not going to come put it in for him either. And I'm certainly not going to certify mine for commercial service, even if it works like a charm for my own purposes.

So he's got a 3 db antenna now. If he's actually getting 1.5 watts ERP as he claims he measured, then he delivering .75 watt to the feed point of the antenna. If he could use a 6db gain antenna, then his ERP will be 3 watts instead of the 1.5 watts he now has.

(I certainly won't deny that the ability to use a field strength meter close to the ground to measure the radiated power of a transmission system which is mounted at a couple of hundred feet is highly suspect at best, but that's the information we have to deal with. It's true that his measured power seems pretty low for the quality of the cable, but I wouldn't trust the measurement too much, myself.)

If we make the assumption that all the transmission losses through the air and stuff that we don't know about from the original post result in a 1 microvolt signal (OK, I picked an easy number, not a realistic number) at the base of the input transistor in the receiver, then doubling the ERP will result in a 2 microvolt signal. If you assume he has a 3db antenna on both ends, then changing them for two 6db antennas will quadruple his signal strength at the receiver. Now if we assume that modifying the receiver inputs to get an extra 6db gain is trivial, then why didn't the receiver manufacturer make it more sensitive in the first place? Probably wasn't trivial if you throw in all the other things we don't know about the system..

So, a better *RECEIVER* antenna will make that single receiver have a better signal, but exactly the same relative upgrade at the transmitter end will make *EVERY* receiver have a better signal.

Of course, I don't know if there is indeed more than one receiver listening to the transmitter. If there is only one, then I'd change whichever end is easier to get to or easier to modify the license. And I'll stick with Curt; the antenna is a lot easier and cheaper than the coax to modify. A VHF antenna is small enough I can still crawl up on top of my house holding one.

Michael
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Michael R. Batchelor - Industrial Informatics, Inc.
Contribute to society: http://www.distributed.net/ogr/
 
it's not really that simple, believe me. Radio and microwave comms equipment design is what I do for a living. The OP was looking to increase his range from 20 to 80km or something like that. At the range he wants, there will be no strong useful signals. It will stretch his link losses budget to the max . A homemade antenna with unmeasured secondary lobes could have very poor directivity resulting in less suppression of local interference. A higher gain omnidirectional is not cheap . A higher gain commercial Yagi will be robust enough but will be longer, will need to be accurately pointed at the target, and maintain its orientation , will basically allow only point to point comms. The mast will need to be stronger to support the longer Yagi . Usually the highest mast is used at the master transceiver site , so there will be only one cable to be optimized. If he can gain 3dB using a better feeder, he will be able to use an antenna with 3dB less gain (than if he only relied on the higher gain antenna and kept his old cable) . The lower gain antenna is smaller , less expensive , not as hard to aim (if it's a Yagi) , less windage , less weight at the top of the mast. The remote stations usually have shorter masts than the master and the problems aren't as expensive there. To quadruple the range to the required 80km he will probably need to use significantly increased gain at both ends of the rf link and minimize feeder losses , depending what link margin his system has now . IF he has 3dB antennas it probably won't be enough for his 80km, he'd have to get some 6-9dB Yagi's . Higher than that they become too cumbersome . This would yield a 6-12dB increase in link gain , not enough to increase range 4 times. He will have to reduce losses and still won't have that much margin left, meaning rain, interference, plus the usual fading mechanisms will make it marginal at best . Trying to do it on the cheap probably won't work. 80km links with cheap equipment are possible but not trivial and I wouldn't take it lightly.

matt tudor
 
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Michael R. Batchelor

mariusrf wrote:
> it's not really that simple, believe me. Radio and microwave
> comms equipment design is what I do for a living.

I believe you. It only a hobby for me.

> The OP was looking to increase his range from 20 to 80km or
> something like that. At the range he wants, there will be no
> strong useful signals. It will stretch his link losses budget
> to the max . A homemade antenna with unmeasured secondary lobes
> could have very poor directivity resulting in less suppression
> of local interference. <clip>

I wouldn't suggest a homemade antenna for a commercial application. Note I didn't offer to make him one. I think the biggest problem with the whole discussion is that we don't know what kind of license the system operates under, so we have no idea what the controlling regulatory agency will allow. Ergo, everything we say is just a guess.

But it seemed to me he wanted the single easiest thing to try, which was what Curt and I were suggesting. The whole installation has a list of problems, not the last of which is that he's used
good HF transmission line in a VHF application. But the line is already run and probably not the easiest thing to change. And it's true that if he wants to quadruple his distance he's got to
4^3 his link gain, which is no small feat.

Overall, I'd suggest to him that if he really wants to solve the problem, find someone who does commercial radio installations for a living, get them to solve the problem for him, then write a
check. That way he can spend his time doing what he gets paid to do rather than chasing his tail at something he's not familiar with. In life, I've found that it's best if I stick to what I
know in my professional life, and call a plumber, carpenter, lawyer, RF designer, or other professional when I need something outside my skills. I make more money working on a problem I
understand than I save messing around with a problem I don't.

--
Michael R. Batchelor - Industrial Informatics, Inc.
Contribute to society: http://www.distributed.net/ogr/
 
You mention only stacked Yagi antennas. Have you considered a parabolic dish with much higher gain?

Dick Caro
 
C
Well, Yes, I missed the line loss part. Who in the world would install a transmission line with losses that high? Either really unsuitable coax or a high VSWR or both. Still, the antenna is often the easiest and cheapest thing to change, given that the rest of the system is at least adequate.

Regards

cww
 
C
Another approach worth considering with this high loss situation would be simply to drastically shorten the transmission line by siting the transceiver much closer to the antenna and using a short low loss feeder. It should be much cheaper to extend the data link to the xceiver and essentially lossless. Control folks should be able to provide the remote switching etc. Then, get a better antenna :^). Find a local amatuer radio club. The older members will know a lot about max ERP for min cost.

Regards.

cww
 
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Peter Whalley

Hi Iqbal,

The power loss ratio will remain the same so if with 25 W in you got say 2.5 W out then with 45 Watt in you will get 4.5 W out. However the increase in power required to extend the range from 40 km to 80 km is likely to be about 16 times and maybe even much more due to the shadowing effect of the curvature of the earth and any particular obstructions on the path (mountains etc).

To get best results, put in much lower loss cable (say Andrews LDF-5-50), increase antenna gain to the maximum extent possible (say an array of 2 or 4 off 6 to 9 element yagis depending on frequency) and fit pre-amplifiers to the receivers if you're in a low noise and low interference area.

If that doesn't work then you could try polarisation diversity. Split your transmitter power between a horizontally polarised and a vertically polarised antenna array at each end and then have similar receive antennas at the other end with each fed into a seperate receiver each of which then feeds a diversity combiner.

The other major factor in the design is the reliablity of the link that you require. Any link will fade out occassionally. If you have a normal level signal that is 10dB above the noise then it will fade out about 10% of the time (90% reliablity), 20dB margin drops this to 1%, 30dB margin 0.1% etc. High reliablity requires very large increases in average signal strength to provide adequate fade margin. ie to go from 90% reliablity to 99% reliablity requires an increase in power of 10 times.

Considering the amount of money you may now be looking at spending, it's probably time to call in a professional radio engineer and get them to do a review of the path to determine just what is going to be required.

Regards

Peter Whalley

Magenta Communications Pty Ltd Melbourne, VIC, Australia
e-mail: peter*no-spam*@magentacomm.com.au
delete *no-spam* before sending
 
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Peter Whalley

Hi Michael,

Gain is not always gain. Increasing receiver gain and reducing receiver feed cable loss increases both signal and received noise hence signal to noise ratio remains constant and may provide no improvement at all. Depends on local ambient noise. Above about 70 Mhz in rural locations, noise level is probably low enough that you can get some improvement at the receive end but try reducing cable loss in an urban 27MHz CB and you get no improvement whatsoever. Increasing receiver gain in this environement may make things worse because of overloading of the receiver front end.

Increasing transmitter level always increases signal but not noise.

Regards

Peter Whalley.

Magenta Communications Pty Ltd Melbourne, VIC, Australia
e-mail: peter*no-spam*@magentacomm.com.au
delete *no-spam* before sending
 
Hi Peter,

The receiver noise is a given constant for a certain receiver . Reducing feed cable losses will increase the signal level at the receiver input, therefore increasing the signal to noise (noise is thermal and generated mostly in the receiver front end).

You are mostly correct in saying rural areas are not as "noisy" as urban sites, except for the fact that the urban noise is not really white noise or any kind of noise, it is just made up of many other transmitters operating at the same time. Some of these are in band, some are on different frequencies , some are continuous, some are pulsed or intermittent, some are close and some are far away. Integrating all these looks like raising the noise floor level , although this is not noise strictly speaking.

Receiver front ends should (and usually do) have enough selectivity due to bandpass filters to reject out of band interference before the interference signal reaches any receiver stage where it can produce intermodulation . It takes a really cheap receiver front end to allow enough out of band interference to generate in-band intermodulation products. In-band or co-channel interference signals are much harder to eliminate if present , although spread spectrum (frequency hoppers especially) do a good job (and so do directional antennas for some types of applications ) . The FCC allows in the USA adaptive frequency hopping in some frequency bands, where the transceivers are sensing the channels with the worst levels of interference and temporarily take them off their hopping table .

Best Regards, Matt Tudor , MSEE http://www.gigahertzelectronics.com
 
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Peter Whalley

Hi Matt,

I wasn't talking about noise generated in the receiver front end but about atmospheric noise (natural and man made) that gets into the receiver via the antenna. Reducing the feeder loss increases both the signal into the receiver and also the noise into the receiver hence no change in signal to noise ratio.

Yes, what I refer to as noise includes on channel interference as well as other sources of man made noise. At the end of the day it reduces the sensitivity of the receiver system so the effect is much the same.

Pre amps placed on the front of the receiver to increase sensitivity often have very little selectivity and are prone to intermodulation in the presense of strong adjacent channel interference. Standard receivers are usually OK as you say except in the most extreme situations.

The original poster didn't indicate where the link is located but given that he's trying to use radio over an 80 km path I'd expect it to be rural otherwise just go to your local Telco and get a land line. If this is the case then a receiver pre amp may well be usefull unless the system is operating at the very low end of VHF (say below 70 MHz where skip could be a problem during certain parts of the sun spot cycle).

I'm not sure about the US but I doubt you'd get a license for spread spectrum or frequency hopping in the VHF band especially if you want to use a 45 Watt transmitter.

Regards

Peter Whalley Magenta Communications Pty Ltd Melbourne, VIC, Australia
e-mail: peter*no-spam*@magentacomm.com.au
delete *no-spam* before sending
 
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