Batboard

I have an odd situation that has got me puzzled here. In short these are my results but I will elaborate more below.
A repeater is placed on the second story of a commercial building, antenna by the window. Operator with a portable radio is standing 10-15ft away from the repeater on the same level. Second operator is walking down the street at ground level walking away from the repeater. Portable-to-portable coverage is GREATER than repeater to portable coverage when talking from inside the building to the operator outside the building. Does this make sense?

Here is the setup, not all "links" in this repeater chain are the best but certainly should outperform a conventional portable radio.

Repeater: Desktrac (utilizing 2 maxtrac radios), Freqs. in use are GMRS 467.6000/462.600 PL 107.2
Duplexer: CELWAVE TDN7407A tuned to the above frequencies.
Jumpers from Duplexer to Repeater: RG-142 (1FT)
Coax to Antenna: 10ft LMR-400 (not the best but for the purpose of testing this should be adequate)
Antenna: Comtelco BS450U Unity Gain

The antenna is mounted on a tripod indoors at a height of 8 ft or so. Operator who is near the repeater is standing, portable radio at eye-height about 6 ft. If the operator in the field keys his radio on simplex he can be heard on the other operator's portable BETTER than the repeater can hear him with the base antenna...!
Here is what I checked so far:
1. Connected antenna straight to RX port of Desktrac, put dummy load on the transmit. Marginally better, same results. This should prove the duplexer is not our of wack and causing problems.
2. TX of the repeater is heard ever so slightly better by the portable in the field than hearing simplex portable to portable.
3. Tried a 5db gain base antenna, mounted 15ft higher than the operator inside of the buidling, and this works better in repeater mode than in simplex mode.

Q. Is the indoor base antenna being 10ft or less away from the repeater causing desense?

abrasowski wrote:I have an odd situation that has got me puzzled here. In short these are my results but I will elaborate more below.
A repeater is placed on the second story of a commercial building, antenna by the window. Operator with a portable radio is standing 10-15ft away from the repeater on the same level. Second operator is walking down the street at ground level walking away from the repeater. Portable-to-portable coverage is GREATER than repeater to portable coverage when talking from inside the building to the operator outside the building. Does this make sense?

Here is the setup, not all "links" in this repeater chain are the best but certainly should outperform a conventional portable radio.

Repeater: Desktrac (utilizing 2 maxtrac radios), Freqs. in use are GMRS 467.6000/462.600 PL 107.2
Duplexer: CELWAVE TDN7407A tuned to the above frequencies.
Jumpers from Duplexer to Repeater: RG-142 (1FT)
Coax to Antenna: 10ft LMR-400 (not the best but for the purpose of testing this should be adequate)
Antenna: Comtelco BS450U Unity Gain

The antenna is mounted on a tripod indoors at a height of 8 ft or so. Operator who is near the repeater is standing, portable radio at eye-height about 6 ft. If the operator in the field keys his radio on simplex he can be heard on the other operator's portable BETTER than the repeater can hear him with the base antenna...!
Here is what I checked so far:
1. Connected antenna straight to RX port of Desktrac, put dummy load on the transmit. Marginally better, same results. This should prove the duplexer is not our of wack and causing problems.
2. TX of the repeater is heard ever so slightly better by the portable in the field than hearing simplex portable to portable.
3. Tried a 5db gain base antenna, mounted 15ft higher than the operator inside of the buidling, and this works better in repeater mode than in simplex mode.

Q. Is the indoor base antenna being 10ft or less away from the repeater causing desense?

Well as all people have learned getting into a repeater operation, it is not easy to get good results the first time.

A few comments are in order here to begin with. First, the use of any LMR type coax cable in repeater operation is not recommended. This is due to it's known problems with intermod and desense in repeater operation. The use od an aluminum foils shield with a copper braid over it is like asking for problems. There are those that say it works well. There are many other cases where it was used on repeaters and the problems didn't go away until the coax cable was replaces.

Second, the Maxtrac receivers are not very well shielded. It is not a good move to have the radio in the near field of the radiating antenna. Your simplest move here would be to use a much longer antenna and move the repeater package further away from the antenna. My guess is that your hearing static on the input receiver with the distance increasing from the portable out on the street.

The placement of the antenna by a window may or may not work well. You have to consider the construction of the building and if the window has metalic particles in it for reduction of sun coming in. If the building has much steel rebar around the window, your antenna radiation patern will be destroyed. Plus you also have to consider the cone of silence around the near field location under the antenna. Those antennas are designed to put the radiation out on the horizon, not DOWN ON THE STREET below it.

Last but not least is the duplexer your using. You may need to do some homework on just how much isolation between the transmitter and receiver a repeater really needs to function well. Depending on who you talk with, the number will vary between 90 and 100 db isolation. Your duplexer only has 70 to 74 db isolation. That may be on the low side depending on how much TX power your trying to run. Again the Maxtrac is not a very good receiver to be using in repeater operation. You would do much better with a Micor receiver.

These are just a few comments that I hope will guide you in your endevors.

Thanks Jim. All of your comment and suggestions are well received, and most are in the works pending available funds. I guess I am just in denial that all things considering, a portable would do better than the repeater, given the same location/conditions. A lot of factors that you pointed out I am isolating an testing to see which are the largest culprits and fix/replace those pieces first.

Thanks for the help!

The antenna sitting within 10-feet of the repeater is likely the main culprit. Think about it...the duplexer's job is to isolate the TX from the RX. By sitting the antenna next to the repeater, you've effectively 'bypassed' the duplexer and are allowing the TX to blow right into the receiver, causing some level of desense. Especially with a Desktrac, which has very little shielding to speak of. Get 20+ feet of vertical separation & you should see a big difference.

That said, have you tested the Desktrac on a service monitor to ensure it's up to spec?

We've had a lot of rptr rentals go bad because the customer put the mag mnt antenna a few feet away from the radio.

Correction - We allowed our rentals dept to hand out mag mnts, and the rest was bound to happen.

Maxtracs are not forgiving of strong fields. I've seen engine cowl TMB antennas kill xmit audio in mobiles, and site noise turn rcvrs into stone.

Also: Your repeater's antenna is likely higher gain, which means most of the signal goes out horizontally, and your HT is closer to an anisotropic radiator, and sends signal downwards. Your "man in the street" may be below the radiation pattern of the repeater antenna but not below the pattern of the HT.

I would ask what your intended purpose for this setup is. Enhanced in-building coverage, extended portable coverage around a few blocks..?

You're on the right path using a unity-gain antenna, but your TX power should be no more than 10 watts. That will solve your desense issue and keep you compliant (or almost) with RF exposure limits.

I appreciate all of the feedback. The biggest concern for me now is the antenna being too close causing the desense. I may try and tune the power output down to around 10watts as suggested, but I think moving the antenna further away vertically will help the most. I have not put the destrac on a services monitor but have used it in the past with good results on a rooftop 10ft mast, 2 stories high. It is due though for a tune-up/check.

The purpose of the repeater is actually for rentals, hence the desire for a small antenna setup. I was looking to extend portable coverage just beyond its limits.

Bill_G wrote: Correction - We allowed our rentals dept to hand out mag mnts, and the rest was bound to happen.

Lol...been there!

Bill_G wrote:We've had a lot of rptr rentals go bad because the customer put the mag mnt antenna a few feet away from the radio.

A few feet? How about stuck to the top of it. I love seeing that lol.

OP, I wouldnt worry about the lmr400, most of its issues were traced to long term outdoor use.