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Author Topic: Help with Field Emcomm Antenna Configuration  (Read 3894 times)
KJ4KLM
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Posts: 19




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« on: December 04, 2012, 12:41:17 PM »

I manage comms for a regional Search and Rescue group.  Being a highly trained but All volunteer organization, comms can be quite challenging  given the LOW budgets we have to work with.  I currently run  a 40FT portable tower with  Diamond X50s at the top.  This antenna services simplex or crossband voice. I feed a Kenwood TM-D710A. Typically, we run simplex on one side of the radio (either UHF or VHF) and I run APRS receive on the other.  We track our Teams live using APRS.  

Recently, we have found more and more applications for Crossband, and thus both sides of the 710a are being used and I can't track APRS. Thus I have added an old TD7a as an APRS receiver-only to connect to the PC tracking software.  

My question is antenna config.  Are there any devices which would allow me to continue to use just the one X50 Antenna to feed the receive of both radios, AND still transmit on either band?  My Guess is no.  Thus I am considering a second, receive only antenna for the APRS receive.  The real question is can I mount both antennas on the same mast and not get too much interference?  I know that is very much frequency based.  We typically use UHF simplex for voice, or crossband UHF and VHF simplex. Thus Typically transmit is only UHF.  My thinking is that I leave the X50 on the top of the mast at roughly 45' on center, and then mount the receive APRS ONLY (144.390) at 20 Ft on a 24 inch arm offset from the mast.  Is this sufficient separation to not blow out the APRS Receiver?  I typically transmit at 15 Watts, but have gone to 50 watts in some areas.  

OK I am a rank amateur in antenna theory so please be kind.  Any help would be appreciated?

Lee
KJ4KLM
« Last Edit: December 04, 2012, 01:15:24 PM by KJ4KLM » Logged
AA4HA
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Posts: 1589




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« Reply #1 on: December 05, 2012, 07:41:31 AM »

Your real challenge is going to be in preventing the transmitters from overloading the receivers of the other radios operating in the same band. You may be transmitting with 40-50 dB of signal (10-100W) and receivers usually work down around -50 to -100 dBm. Even if you did not blow out the front end of the receiver (RF amp) you are definitely going to be driving things into non-linear operation (think of intermodulation across vast swaths of spectrum).

There are things to attach multiple receivers in the same band (multicouplers) and even multiple transmitters in the same band (cavity filters and circulators). Doing both at the same time is really a challenge, especially if you want to operate in the same band with one antenna.

If you were using different bands (2 m and 70 cm or other combinations that were exclusive) you could use diplexers (band/antenna combiner/splitters). Still you get into issues with harmonics and normally you would want some additional filtering.

A better choice would be to use multiple vertically polarized antennas, separated vertically on a mast. You would end up putting each antenna in the "null" of the radiation pattern of the antenna above or below it. The separation would need to be at least one wavelength apart (really >10') on the mast and you still are going to be hitting the receivers with pretty high signals (feedline imperfections, reflected signals, etc...).

If you had the means of limiting a transceiver to a single frequency you could use a filter with an extremely high "Q factor". I have had to do that before on 900 MHz M.A.S. systems where there were many transmitters on the same building, some separated by only a few feet and within 100 KHz of each other. I have no relationship with the company and only am an astounded customer.

http://www.qfilterproducts.com/ultra-q.html

It is expensive (few thousand dollars) but it is a programmable cavity filter (USB port) and does indeed allow for close channel spacing (down to 6.25 KHz). If I had a repeater that was desensing on the front end it would be near the top of my list for a solution.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KJ4KLM
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Posts: 19




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« Reply #2 on: December 05, 2012, 08:40:19 AM »

Thanks for replying, Tisha. What you describe in your 4th paragraph, so much more clearly than I did, is what  I hope to accomplish. The  Diamond X50a is a vertically polarized dual band antenna. I was considering a similar antenna for the Receive Only APRS antenna (though maybe not as big and not dual band).  I was planing on mounting it as you describe, but 20' below the top antenna. Since the APRS receive only Antenna will only be on 144.390 Mhz, and the Crossband antenna will be on 146 +- Mhz and 444+- Mhz my question is, how much interference or intermod could I expect and how could I filter/compensate for that?

Also, which antenna would be better placed on top.  The Crossband, or the APRS Receive only antenna? I am leaning toward the Crossband.

Lee
 
« Last Edit: December 05, 2012, 08:45:37 AM by KJ4KLM » Logged
WX7G
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Posts: 6214




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« Reply #3 on: December 05, 2012, 08:46:04 AM »

All this can be modeled using NEC to find a best position for the antennas.

Another way to gain isolation is to tilt each antenna 45 degrees from vertical. One antenna is tilted to the left, the other to the right. The antennas will "lose" 3 dB of gain in both transmit and receive. Good common-mode isolation of the feedlines will help and a NEC model will tell us if the nearby vertical mast defeats the hoped for isolation.
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WB6BYU
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Posts: 13485




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« Reply #4 on: December 05, 2012, 09:02:07 AM »

You can run TX and receive simultaneously on the same antenna - that's what most
repeaters do all the time.  With 600kHz spacing on 2m they usually have a 4- or
6-cavity filter to separate the two frequencies.  You might not need that much
with 2 MHz spacing, but it still isn't a trivial exercise, and it requires very good
shielding all around to prevent the TX from desensing the RX.

If you aren't transmitting all the time, you probably can manage with occasional
lost APRS packets, so desense wouldn't be as much of a problem.  You might be
able to get by with a single cavity (or a few helical resonators) in each side as
a cheaper solution.


My experience operating mobile is that my rig would desense when I got within
about 2 car lengths of another station if we were talking through a repeater.
(Because the repeater signal is fairly strong, there may have been some degradation
of weak signals before that.)  Stacking vertical antennas one above the other
gives better isolation, as long as they are decoupled from the mast and coax,
so with a second antenna 20' lower you may find that performance is acceptable
without any additional filtering.  None of my mobile rigs have been damaged by
the car parked beside me transmitting, and similarly I've been in rigs with 10
different radios that didn't suffer damage either (though the desense was
pretty bad.)

So the real question is whether desensing the APRS receiver when you transmit
on 2m causes too much disruption and loss of data:  if you can live with it, it
makes the system design much easier.  If not, then you may need additional
high-Q filters and good decoupling of the coax and mast to make it work.
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KJ4KLM
Member

Posts: 19




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« Reply #5 on: December 05, 2012, 10:07:47 AM »

All this can be modeled using NEC to find a best position for the antennas.

Another way to gain isolation is to tilt each antenna 45 degrees from vertical. One antenna is tilted to the left, the other to the right. The antennas will "lose" 3 dB of gain in both transmit and receive. Good common-mode isolation of the feedlines will help and a NEC model will tell us if the nearby vertical mast defeats the hoped for isolation.

Thanks for the repsonse. I wouldn't even know where to begin with NEC modeling. Any suggestions on an easy way to do this without learning all the electrical engineering to populate such programs. As for Common Mode Isolation of feedlines and Mast. how is this best achieved.  I did say I was a total amateur about antenna theory and the tools required.

Lee
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KJ4KLM
Member

Posts: 19




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« Reply #6 on: December 05, 2012, 10:24:25 AM »

You can run TX and receive simultaneously on the same antenna - that's what most
repeaters do all the time.  With 600kHz spacing on 2m they usually have a 4- or
6-cavity filter to separate the two frequencies.  You might not need that much
with 2 MHz spacing, but it still isn't a trivial exercise, and it requires very good
shielding all around to prevent the TX from desensing the RX.

If you aren't transmitting all the time, you probably can manage with occasional
lost APRS packets, so desense wouldn't be as much of a problem.  You might be
able to get by with a single cavity (or a few helical resonators) in each side as
a cheaper solution.


My experience operating mobile is that my rig would desense when I got within
about 2 car lengths of another station if we were talking through a repeater.
(Because the repeater signal is fairly strong, there may have been some degradation
of weak signals before that.)  Stacking vertical antennas one above the other
gives better isolation, as long as they are decoupled from the mast and coax,
so with a second antenna 20' lower you may find that performance is acceptable
without any additional filtering.  None of my mobile rigs have been damaged by
the car parked beside me transmitting, and similarly I've been in rigs with 10
different radios that didn't suffer damage either (though the desense was
pretty bad.)

So the real question is whether desensing the APRS receiver when you transmit
on 2m causes too much disruption and loss of data:  if you can live with it, it
makes the system design much easier.  If not, then you may need additional
high-Q filters and good decoupling of the coax and mast to make it work.

Thanks for the reply. IF there is little risk of damaging the APRS Receiver on 144.390 then I am going to simply try the APRS Receive only antenna 15-20' below the Dual Band Transceiver antenna at the top of the mast (45'). A missed packet here and there will likely not make a difference.  Can you explain a bit more about how one goes about achieving "good decoupling of the coax and Mast"?  Does it require High Q filters, shielded cable of some kind?  special grounding. The mast is grounded well per military specs on most sites it is used. The Coax is Belden 9913 run through a IEC lightning arrestor which is grounded to the Milspec Grounding array.  The grounding is 4 quarter inch stainless steel cables, 25' long each, laid on the ground level, each with 5, equally spaced, 15-18" deep copper clad grounding rods, bonded with Bronze clamps.

Lee
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WB6BYU
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Posts: 13485




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« Reply #7 on: December 05, 2012, 10:47:34 AM »

Quote from: KJ4KLM
... Can you explain a bit more about how one goes about achieving "good decoupling of the coax and Mast"?  Does it require High Q filters, shielded cable of some kind?  special grounding? ...



No, it isn't anything about how the mast is grounded.  That has little effect on VHF
because it is several wavelengths from the antenna to ground.

What you need to do is to make sure that the antenna currents flow only on the
antennas and not on the masts or the outside of the coax.  For example, if the
coax from the upper antenna runs down the mast within a foot or two of the lower
antenna, and there is any common mode current flowing on the outside of the shield,
then the lower antenna will pick up that signal, which may be stronger than that
radiated from the upper antenna itself.  Most J-poles, for example, are prone to
common mode currents and would be a poor choice for the upper antenna for that
reason.  Similarly, if the coax to the lower antenna picks up signals then it may
not matter how precisely the lower antenna is positioned in the null below the
upper antenna because more RF will be picked up on the coax.  Currents flowing on
the mast can cause the same sort of coupling between the antennas.

Ferrite beads on the coax may help, as will spacing the antennas further out
from the mast, though that can cause some mechanical problems.

Also be aware that the coupling depends on the distance between the two
antennas, not between the mounting points.  A 15' antenna mounted 20' below
the upper one only has 5' of distance between them.

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KJ4KLM
Member

Posts: 19




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« Reply #8 on: December 05, 2012, 12:06:02 PM »

Quote from: KJ4KLM
... Can you explain a bit more about how one goes about achieving "good decoupling of the coax and Mast"?  Does it require High Q filters, shielded cable of some kind?  special grounding? ...



No, it isn't anything about how the mast is grounded.  That has little effect on VHF
because it is several wavelengths from the antenna to ground.

What you need to do is to make sure that the antenna currents flow only on the
antennas and not on the masts or the outside of the coax.  For example, if the
coax from the upper antenna runs down the mast within a foot or two of the lower
antenna, and there is any common mode current flowing on the outside of the shield,
then the lower antenna will pick up that signal, which may be stronger than that
radiated from the upper antenna itself.  Most J-poles, for example, are prone to
common mode currents and would be a poor choice for the upper antenna for that
reason.  Similarly, if the coax to the lower antenna picks up signals then it may
not matter how precisely the lower antenna is positioned in the null below the
upper antenna because more RF will be picked up on the coax.  Currents flowing on
the mast can cause the same sort of coupling between the antennas.

Ferrite beads on the coax may help, as will spacing the antennas further out
from the mast, though that can cause some mechanical problems.

Also be aware that the coupling depends on the distance between the two
antennas, not between the mounting points.  A 15' antenna mounted 20' below
the upper one only has 5' of distance between them.



Very clearly explained!!  Thank you!  I can certainly try ferrite beads.  I can also try to offset from the mast by Putting the two antennas on PVC Arms.  How far from the mast would they have to be? When ti comes to vertical stacking, do the two antennas have to be DIRECTLY above each other (each on the same length arm) or can the one on top be directly on the top of the mast and the one on the bottom (receive only) of the stack be offset by maybe 18-24 inches.  Offsetting both to the same side of the mast may cause come pinching in the Mast raising mechanism but I think I can work around it with helpers on the guys lines while raising and lowering.

Lee
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AA4HA
Member

Posts: 1589




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« Reply #9 on: December 05, 2012, 05:40:47 PM »

A collinear antenna (vertical with gain, typically looks like a fiberglass stick) will have a radiation pattern that looks like the cross section of a doughnut when you look at it from the side. http://www.boat-project.com/tutorials/vhfant.htm The area of negative gain (null) is pretty large. I would not worry too much about getting them exactly one above the other. The most important thing will be in maximizing the spacing between antennas.

With all side-mounted omni antennas you do want to try to keep some offset from the mast as being close to metal really messes up the radiation pattern. Instead of it looking like a nice circle from above it can look like an ellipse or a keyhole pattern. That all depends upon the spacing and is another complex area of antenna modeling that I will not go into.

For the applications I deal with (commercial stuff, public safety) I try to keep verticals stood off a few feet away from the tower. That may not be practical on an EMCOMM crank up mast or for a lighter structure where torsional forces from the wind can cause the structure to want to corkscrew into the ground.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KJ4KLM
Member

Posts: 19




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« Reply #10 on: December 06, 2012, 02:30:01 PM »

Tisha, Thank you again for the reply. The x50A is a collinear as you describe. Since the Antenna which will be place on the mast beneath it will only be a receive only antenna, is the radiation pattern of the second antenna even an issue?  You are right about the concerns of the mast in the field.  Wind will be factor.  The mast is a simple 3" aluminum sectional mast. Wind will likely spin the pvc arm to a downwind position, but since each section spins fairly freely, only restricted by the friction of the connecting joins the corkscrew into the ground is not a concern.  The mast also sits on 12 inch pad to prevent ground peneration. However I think I can guy the arm to maintain a down wind position.  The second antenna will be a collinear vertical as well but a shorter one, an X30a. It should have a fairly low wind cross section.  

If no one else has additional advice I will try this configuration and see what kind of results I achieve. I will report back when I have completed this.

Lee
« Last Edit: December 06, 2012, 02:37:09 PM by KJ4KLM » Logged
N4CR
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Posts: 1694




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« Reply #11 on: December 06, 2012, 06:46:06 PM »

Lots of good information presented already.

The commercial components that it takes to really keep a transmitter from desensing the nearby receiver are expensive. The entire goal might be impossible if the radio itself it not shielded well enough to block out high level RF in the vicinity. Great shielding is one of the reasons that commercial gear is expensive. Band Pass filters are expensive and some types of them are large as well. Double shielded coax might be required.

I will suggest that your least expensive route will be to spend a small amount of money for another 40' support and get those two processes away from each other. If your APRS receiver is not well shielded, it might be the only thing that works.

Another option might be to acquire a mobile repeater cavity and only use half of it by putting a dummy load on the TX port and put the RX side on the APRS receiver with the notch set to the transmit frequency of the other transmitter. I don't find a good source for these on the internet. Most of these are UHF but there are VHF ones available at retail, such as this one.

http://www.radiotwoway.com/cart/index.php?target=products&product_id=15741

As you can see, commercial equipment is expensive. And no guarantee it would work.

Notch filter cans will dramatically reduce the necessary separation distance. (once again, assuming your APRS receiver is well enough shielded)
« Last Edit: December 06, 2012, 07:01:06 PM by N4CR » Logged

73 de N4CR, Phil

Never believe an atom. They make up everything.
KJ4KLM
Member

Posts: 19




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« Reply #12 on: December 07, 2012, 05:42:52 PM »

Lots of good information presented already.

The commercial components that it takes to really keep a transmitter from desensing the nearby receiver are expensive. The entire goal might be impossible if the radio itself it not shielded well enough to block out high level RF in the vicinity. Great shielding is one of the reasons that commercial gear is expensive. Band Pass filters are expensive and some types of them are large as well. Double shielded coax might be required.

I will suggest that your least expensive route will be to spend a small amount of money for another 40' support and get those two processes away from each other. If your APRS receiver is not well shielded, it might be the only thing that works.

Another option might be to acquire a mobile repeater cavity and only use half of it by putting a dummy load on the TX port and put the RX side on the APRS receiver with the notch set to the transmit frequency of the other transmitter. I don't find a good source for these on the internet. Most of these are UHF but there are VHF ones available at retail, such as this one.

http://www.radiotwoway.com/cart/index.php?target=products&product_id=15741

As you can see, commercial equipment is expensive. And no guarantee it would work.

Notch filter cans will dramatically reduce the necessary separation distance. (once again, assuming your APRS receiver is well enough shielded)

N4CR, Great Info for me to consider. Using RG-214 for the APRS antenna is no problem and probably a very good idea. I will have to further into Bans Pass filters. Perhaps there are some designs out there to build a small unit which will suffice. Or , if I am lucky I might find something in the used market.  If I bought another mast, it is my understanding that the antenna would have to be located potentially hundreds of feet apart. Any idea what a reasonable distance apart might be if the receiver is listening on 144.390 and the transmitter is roughly 147Mhz?

Getting another mast is certainly cheaper than many options, if I can have them located within a reasonable distance of our Command post.  I guess I am being a bit lazy by not wanting to have to erect, guy, and ground two masts, each time we setup, and then break it all down again.  However, I might just be lazy enough to choose and electrically engineered solution, rather than a mechanical one.
« Last Edit: December 07, 2012, 06:17:20 PM by KJ4KLM » Logged
KJ4KLM
Member

Posts: 19




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« Reply #13 on: December 07, 2012, 07:49:10 PM »

Phil,  Will This sort of band pass filter do the trick?  I think I can get one for $100.

http://www.dci.ca/pdf/DCI-145-2H.pdf

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N4CR
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Posts: 1694




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« Reply #14 on: December 08, 2012, 02:08:02 AM »

Phil,  Will This sort of band pass filter do the trick?  I think I can get one for $100.

http://www.dci.ca/pdf/DCI-145-2H.pdf



It's only ok if you can tune it. I don't see any indication that it's tunable.

That filter shows about 50 db isolation at 5mhz. If you can tune that to the APRS frequency, you might be able to get your two antennas quite a bit closer. Two of them would supply 100 db isolation and would likely make it so you could mount both antennas one under the other.

Why are you grounding your temporary masts? Seems like a total waste of time to me.

The only way you will know for sure how far the antennas need to be separated is to experiment.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
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