Will a passive antenna work?

[airboss20]

Reliable portable radio comms in our station has always been difficult. The building construction is lightweight steel truss with tin wall studs and corrugated metal decking as the base for the roof.

Currently the Police operate on VHF and the FD is dispatched on VHF. All dispatch, operations and tactical channels operate between 154 Mhz and 155 Mhz. With repeater input offsets on 156 Mhz. PD is P25 and FD dispatch is analog.

The Minitor VI VHF pagers are unable to receive traffic clearly, and the portable radio coverage inside is spotty. More of an issue transmitting than receiving.

I've heard of people deploying passive antennas with a yagi on a radio tower directed at the closest repeater and an omni-directional antenna mounted from the ceiling inside.

Before investing in the project, I would like to know if anyone here has attempted such a solution or has advice.

 

[lenk911]

I have engineered many of these and the key is to pay attention to the laws of physics. For a passive system there are four losses that subtract from the gains.
1. The propagation and ambient losses between the host radio site and the outdoor passive antenna. If it is optical with Fresnel clearance then it is free space losses plus ambient. Otherwise it is worse.
2. The losses within the passive repeater itself. Usually it is the transmission line.
3. The path losses between the indoor antenna and the receiver.
4. Noise: Noise from manmade devices can be as high as 25 db on VHF.
Add to that the system gains of the host, passive antennas and the receiver.
s
Most people mess up with #1 and 3. If you are a mile from the host, free space loss is -80 db at VHF. If you are 20 feet between the passive antenna and the receiver that can be another-30 db. Usually the gains are not enough to overcome losses #1and #3 unless you put an amplifier in the passive antenna system. Best to have the detailed engineering performed before you buy! You may have a decent signal but are plagued by manmade noise in the building.

 

[WA0CBW]

Like lenk911 said. Simply putting up two antennas and connecting them with coax is unlikely going to work. Even following the technical requirements stated it is rarely successful.

Bill

 

[mmckenna]

Yeah, passive isn't going to work.

The way this is done is a "Bi-Directional Amplifier". It's a donor antenna on the roof feeding an amplifier. The amplifier feeds antenna(s) inside the building. The system works in both directions.

1. No, you cannot build this yourself with parts off Amazon.
2. It needs to be very carefully engineered by someone who had designed/built BDA systems before.
3. They need to be carefully set up as to not self oscillate. Self oscillation will not only not make it work, it'll wipe out the repeaters for everyone else and cause all kinds of interference issues for others in a wide area.

The solution to your issue requires a knowledgeable engineer. (Notice I said "engineer" and not "hobbyists", "ham radio operator", etc…)

 

[lenk911]

BTW I second MMCKENNA above. You also need permission of the radio system owner to install a BDA on their system and follow FCC reporting rules. The NFPA also has standards on BDA systems you need to follow in addition to NEC and building codes.

 

[speedway_navigator]

We used a passive repeater in a hospital to get paging in the basement, that was a 300W xmtr feeding hardline. I used one in the super dome with the repeater in the building, not much path loss. That was 2 yagis with about 10' of LMR400.

I made this form for you to calculate how much signal strength you might have inside your station with a passive repeater, I hope the experts here will double check my math.

https://tech.tacticalrf.com/link_budget.xlsx

As lenk911 noted this is for ideal conditions like in an RF test chamber. You fill in the yellow boxes and my math hopefully will do the rest.

Line 18 is lenk911's estimate, the path loss formula says 40 dB @ 50'.
Line 19 is where extra losses go - drywall, doors, metal filing cabinets, turnout gear, cops hanging around for free donuts, etc, it all adds up.

Line 21 is an estimate, most radios will open squelch at -120 dBm but that is coax into the antenna port, your antenna will attenuate based on damage from flexing and wear. The portable might detect a signal at -120 but not have enough strength to recover the intelligence after demodulation.
The more margin the better chance you have of good comms.

Uplink, the technical term from you to the tower site, is the weak link as you noted. Put your portable in the center of the hood or roof of a car or SUV and see if dispatch gets better reception.

We put a paging xmtr in a station and coverage was great. While we were there the fire chief started replacing the old light ballasts with the new green energy type. We got a call the next day they lost pages in the station but the pagers worked outside. The RFI from the new ballasts was enough to block the signal.

 

[lenk911]

I would make some changes to your spreadsheet.

1. You have hidden the space loss calculations but the 36.6 gives it away. Rarely on VHF do you have 0.6 Fresnel zone clearance even if you can optically see the antenna, especially at 10 miles. On VHF, Bullington requires the use of Plane Earth Loss in these instances rather than free space loss. This will increase your path loss by at least 6 db.

2. The end signal level you have calculated is at 50% reliability due to varying K factors. Increase its confidence by adding Rayleigh fading. i.e. 90% = 10 db and 95% = 15 db margin.

3. The donor antenna at 3 db is not going to be 3 db on all planes. Its going to have lobes and nulls. Unless it is looking down a narrow hall and the receiver cannot get out of the main lobe, it should be de-rated--may be it was, if so, sorry

4. You show zero for additional loss. There is practically nowhere in the world where VHF does not have some noise losses. Interior losses, as you observed from ballasts, can range up to 25 db on VHF. Buck converter power supplies (aka wall warts) are good for 3-10 db.


Many people see passive antenna engineering as black magic. Rarely do they work because of all the factors that affect their performance. Congratulations on yours being successful!

 

[speedway_navigator]

I left the additional loss at 0 for the OP to fill in. Plane earth loss is a new term to me, thanks for the info.
I just wanted to give the OP an idea of all the losses he's facing.

The UHF repeater only worked for about 50' down a hall, when the Bearcom guy did the site survey the door to that hall and the rooms off it were closed for a meeting. Once I setup and tested the main walkway I thought we were in good shape, then the NBA folks moved into the rooms off the hall where the had no uplink.

I shot myself in the foot after I did a site survey at the Phoenix convention center, the NBA needed coverage on the loading dock. I did a test and it was fine, but the dock workers had the nerve to close all the metal overhead doors when they left at 5.

 

[krokus]

How does using leaky coax, instead of an internal whip, affect the coverage & computation?

 

[lenk911]

Leaky coax or trade name = Radiax is a coaxial cable with slots cut in two sides of the cable. The slot size determines the amount of energy radiated and the amount of loss enroute the cable. It is sensitive to mounting--obviously you never want the slots against the mounting object (wall etc). They have minimal radiation along the cable compared to a whip antenna. Slotted cable is best used in tunnels or mines where the distance from the cable to the limits of desired service is small.

If you have a large area to cover, a solid cable interconnecting periodic antennas, called a distributed antenna system is a better choice. At the drops you can control by choice of interface to the cable how much energy you want to drop at a specific antenna. You can also add to the system, if you have enough remaining system gain, to fill in areas that need attention found lacking after installation.

Both can be supplemented with amplifiers enroute. If you use amplifiers, you'll need to follow FCC and NFPA rules as I outlined above.

 

[MUTNAV]

This might help with some ideas.

Thanks
Joel

 

[speedway_navigator]

We ran Radiax for a paging system in a hospital in the pipe chase. I found this antenna mounted by a zip tie, imagine how bad the return loss is being next to 2 metal pipes.

 

[mmckenna]

We ran Radiax for a paging system in a hospital in the pipe chase. I found this antenna mounted by a zip tie, imagine how bad the return loss is being next to 2 metal pipes.

Ah, it's the old "My invoice will be paid before they find this s**tshow of an install" trick...

 

[MUTNAV]

We ran Radiax for a paging system in a hospital in the pipe chase. I found this antenna mounted by a zip tie, imagine how bad the return loss is being next to 2 metal pipes.

Yikes... What hospital has construction like that ! ? It looks like a house built in the 1920s (based on the wood floor and walls).

I thought hospitals had seriously strict building and maintenance codes... Even in the military, there were people that took care of regular buildings and a separate group of people that took care of the hospitals due to the strict and different codes. Just think about the difference in power connectors (hospital plugs vs. regular molded plugs).

Thanks
Joel







Thanks
Joel