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Amateur Radio Antennas For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

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Old 09-05-2013, 7:03 PM
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Default UHF Vertical Seperation for link radio

I have a repeater at location A on 442.950+. I was thinking of linking a repeater at location B on 442.650+(25 Watts). My idea is to link using locations A input/output (442.95/447.95) at location B. Repeater antenna at location B is a omni(gain unknown) and link antenna would be a yagi pointed towards location A directly below the omni antenna somewhere on the tower.

How much vertical separation would be needed to make this work without any additional filtering?

Tim
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Old 09-05-2013, 7:47 PM
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So, you want to operate a radio on the mobile side of the frequency pair, 300 khz away from the co-located repeater, without extra filtering? This is a setup to fail.
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Old 09-05-2013, 8:01 PM
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Wirelessly posted (Moto Droid Bionic: Mozilla/5.0 (Linux; U; Android 4.1.2; en-us; DROID BIONIC Build/9.8.2O-72_VZW-22) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Mobile Safari/534.30)

You might be able to do it using another band for a link frequency. Otherwise zz0468 is spot on IMHO. If you use a link frequency, I would suggest you check and work with your area frequency coordinating committee. Just a thought.
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Old 09-05-2013, 8:02 PM
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Darn it. Mobile RR site double-posting again. Sorry
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Old 09-05-2013, 9:34 PM
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I know of someone doing this, with the link antenna 50ft. directly below the repeater antenna. They are only using 20 watts on the link.
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Old 09-06-2013, 2:45 AM
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Quote:
Originally Posted by profiledescent View Post
I know of someone doing this, with the link antenna 50ft. directly below the repeater antenna. They are only using 20 watts on the link.
I'm guessing there is additional filtering, and they're not just relying on antenna spacing.
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Old 09-06-2013, 7:59 AM
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Originally Posted by profiledescent View Post
I know of someone doing this, with the link antenna 50ft. directly below the repeater antenna. They are only using 20 watts on the link.
I have tried looking for a calculator, but haven't found anything useful.

Tim
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Old 09-06-2013, 8:51 AM
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Quote:
Originally Posted by timkilbride View Post
I have tried looking for a calculator, but haven't found anything useful.

Tim
Here you go: rfcalc1. See calculation item 11 on the spreadsheet.

Please keep the great man of communications, Jack Daniel, KD6YVL-SK, in your thoughts as you use it. He put this out there for everyone.
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Old 09-06-2013, 10:42 AM
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This is being done here in eastern n.c. for some time now.
Here is a link to site information.
443
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Old 09-06-2013, 6:06 PM
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Quote:
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Here you go: rfcalc1. See calculation item 11 on the spreadsheet.
This will tell you how much isolation you can get, but not how much isolation you need.

What is the transmitter noise level 300 KHz from carrier frequency, and on the receiver frequency? If you don't know that value, then calculating the isolation is pointless. You don't know what the target value needs to be.
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Old 09-06-2013, 6:07 PM
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Quote:
Originally Posted by KC4YIN View Post
This is being done here in eastern n.c. for some time now.
Here is a link to site information.
443
I didn't see any data on how their linking is actually accomplished.
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Old 09-06-2013, 7:32 PM
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FWIW, I have seen several of this type of linking, and they have all failed. They were changed over to 220mz and have now been working with no problems for over 10 yrs.

Use yagi antennas when doing the install.
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Old 09-09-2013, 7:40 AM
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Quote:
Originally Posted by zz0468 View Post
This will tell you how much isolation you can get, but not how much isolation you need.

What is the transmitter noise level 300 KHz from carrier frequency, and on the receiver frequency? If you don't know that value, then calculating the isolation is pointless. You don't know what the target value needs to be.
I suspect I'm preaching to the choir, but I'll put it out there for everyone's read - adjacent channel power ratio (ACPR) is determined by two things: transmitter spectral purity and receiver filtering. Not knowing the specifics of Tim's setup in terms of measured response, it's not possible to derive a threshold figure. There are a couple of tactics he can explore. That being sharp bandpass filtering on the transmitter output (which could potentially attenuate half or more of the output power), and sharp bandpass filtering on the linked receiver, which stands to reduce the signal level and impact the systems' link budget. Additionally, receiver IF filtering should be as sharp as possible, as well. Digital capable equipment tend to have much better filtering that is aided by DSP. But not everyone goes out and buys a Quantar or MASTR-III. My experience in helping people build and maintain ham repeaters has been that many will put two mobiles together with a talking controller and a modest duplexer and take great pride in doing so (mostly rightfully so, but that should be the beginning of the learning process). Those devices may be excessively broad and noisy because they are designed to work in as wide a bandspread as possible (450 - 470 or greater), while a repeater is designed to be on one frequency only, until it's re-frequed and realigned. I also have to mention the antenna. Just about every ham grade antenna is not built to commercial/public safety specifications. Things like construction techniques could affect performance (i.e., creating non-linear junctions and an environment for passive intermod [PIM]), especially in RF dense environments.

So, there is no short answer to that without much more information and testing. If I were doing that, I would shoot for as much isolation as I could get, use dissimilar CTCSS tones, and then experiment. But I wouldn't put any stake into reliability until I had some solid experience with the two systems in operation.
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Old 09-15-2013, 7:38 PM
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Put one vertical directly under the other.
Should be fine.
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Old 09-16-2013, 11:11 PM
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Quote:
Originally Posted by Wyandotte View Post
Put one vertical directly under the other.
Should be fine.
Re-read the OP, and then the first couple of replies. The OP is wanting to operate two transmitters, and two receivers, with each pair having the opposite pair's transmitter only 300 KHz away in frequency from it's receiver, with no external filtering, on UHF, using vertical separation alone.

The repeater transmitter can be adequately isolated with sufficient vertical spacing, but that's with a 5 MHz separation from the associated transmitter. But it will also have the link transmitter only 300 KHz away.

This can be done, but it would take a rack full of cavities. There is insufficient data to calculate the spacing requirements, but an educated estimate says that it would take several hundred feet of vertical spacing, AND some cavities. 300 KHz away at 450 MHz is very tough to isolate.
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Old 09-20-2013, 6:02 PM
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You didn't mention how high your station is. Is it possible to lower the power you are using and still make a good link ?
That would be of some help, but you are still going to follow the above suggestion if it's going to work at all.

73's John
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Old 09-20-2013, 6:10 PM
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Quote:
Originally Posted by dksac2 View Post
You didn't mention how high your station is. Is it possible to lower the power you are using and still make a good link ?
That would be of some help, but you are still going to follow the above suggestion if it's going to work at all.

73's John
The repeater will be around 60' AGL. Link antenna will be 6-8' AGL. We haven't tried anything yet. I did find a couple of UHF cavities we may be able to throw onto the link radio to keep it from getting desence. We have to keep the first repeater running first before we can even try to add this second one

Tim
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