![[Lower view of repeater in foreground, duplexer in the background, digipeater on to top of duplexer]](vhf-fm-repeater-lower.jpg)
![[Upper view of repeater in foreground, duplexer in the background, digipeater on to top of duplexer]](vhf-fm-repeater-upper.jpg)
![[View of repeater cable routing]](vhf-fm-repeater-routing.jpg)
![[An entrance view of the utilities of the 3rd level enclosure on the IDRL gantry]](idrl-3rd-level-enclosure-entrance.jpg)
![[The 3rd level enclosure on the IDRL gantry]](idrl-3rd-level-enclosure-front.jpg)
![[A right side view of the 3rd level enclosure on the IDRL gantry]](idrl-3rd-level-enclosure-side.jpg)
![[A top-looking-down view of the 3rd level enclosure on the IDRL gantry]](idrl-3rd-level-enclosure-down.jpg)
![[View of repeater antenna]](vhf-fm-repeater-antenna.jpg)
Select each picture to see a larger view and accompanying text description.
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The repeater cabinet placard shows:
Additionally the internal components are labeled:
Service accessories include:
Here are pictures taken after the repeater refurbishment was completed in June 2002.
Operational with serious intermodulation problems. Improvements are forthcoming.
20 August - A decision was made to change the PL frequency to 173.8 Hz. This was done as a result of several members/attendees not having PL 97.4 or 74.4 Hz PL in the more vintage radios. The SERA guidelines detail that 74.4, 97.4, 131.8, 173.8, and 225.7 Hz are to be used in eastern VA. Website research indicates the PL frequency of 131.8 Hz is in common use in North Carolina for linking repeaters so it seemed best to pick the next higher PL frequency. It seemed from the consensus of the meeting that no one was lacking 173.8 Hz. This higher frequency may turn out to be and advantange since this may permit faster latchup on the PL board since only about 10 cycles are required to obtain a positive PL lock. The downside of using 173.8 Hz PL is that the mixture of the PL tone and male voice frequencies that are strong in the 150-300 Hz range causes a grainess to be heard in the voice patterns for most male users.
A feature is available of pressing "67" sequentially on your tone pad to defeat PL access for 3 minutes. The PL reset timer begins counting from the last time COR access is accomplished. Unfortunately with the intermodulation so severe the repeater receiver is never allowed to rest for at least 3 minutes. Therefore, this function is currently disabled.
On 30 August a conversation with the LaRC frequency coordinator revealed a more complete list of radio system frequencies in use on the IDRL gantry. They are:
| User | Designation | Transmit Frequency | Useage |
|---|---|---|---|
| Amateur Radio | VHF FM Repeater + Sinclair Duplexer | 146.94 | Infrequent, 5 cycles/hr. |
| USG | VHF FM Repeater + Sinclair Duplexer | 165.85 | None detected |
| Army Corp. of Engr. | Unknown | Unknown | None detected |
| USG | Support Net | 170.35 | None detected |
| USG | Pager | 170.4 | Frequent, 20 cycles/hr. |
| FAA | Aviation Net | 172.95 | None detected |
| USG | Safety Net | 173.6125 | None detected |
| USG | Inspection Net | 173.6875 | Infrequent, 5 cycles/hr. |
| LMR Net | LMR1 LMR2 LMR3 LMR4 LMR5 LMR6 LMR7 LMR8 LMR9 LMR10 | 409.35 406.75 408.75 407.15 407.95 408.55 408.95 409.15 406.55 409.95 | At any one time at least 3 frequencies are busy. |
| Business | CH1 | 457.65 | Frequent, 100 cycles/hr |
| WHRO | S-Band Link | 2500-2542 MHz | Unknown, but assumed to be continuous. |
A 3rd order intermodulation problem exists with the repeater transmitter and an adjacent Motorola ASTRO SmartNet communications system referred to in the following paragraphs as a Land Mobile Radio (LMR) system. This LMR system operates on 10 frequencies in the 405-410 MHz band. The repeater receiver is receiving a digital audio signal that sounds like the signals heard from the LMR system on 146.34 MHz when all the following are true:
The problem is further complicated since a total of five pairs of transmitting frequencies of the LMR system are 600 KHz apart. Any single, partial or full combination of the 5 pairs of LMR frequencies may be active at any one time. It has been observed the more pairs of these frequencies are active, the stronger (and less clear) the intermodulation signal becomes on 146.34 MHz.
The interference has been identified as 3rd order intermoduation distortion. It is the combination of the Amateur Radio signal on 146.94 MHz and the difference of any two of the LMR pairs of frequencies that are 600 kHz apart. If any one of these three signals cease, the intermodulation signal appearing on 146.34 MHz also ceases. Since repeater operations require the 146.94 MHz signal to be continuously present, it is a contributing factor, but not a controlling factor in the generation of the intermodulation signal.
Careful monitoring of the LMR system reveals that 10 frequencies are used. Characteristics of each of the 10 frequencies assigned to the LMR system have been observed as follows:
| Channel | Frequency | Purpose | Comment |
|---|---|---|---|
| C1 C2 C3 C4 | 409.35 406.75 408.75 407.15 | Control/Data | Channels C1-C4 are alternating control channels of the LMR communications system. Only one channel will operate as a control channel at a time. The other three function as regular digital data channels. The control channel increments at times to distribute the duty cycle since this channel transmits continuously. |
| C5 | 407.95 | Data | Intermittent, frequently used. |
| C6 | 408.55 | Data | Intermittent, frequently used. |
| C7 | 408.95 | Data | Intermittent, rarely used. |
| C8 | 409.15 | Data | Intermittent, rarely used. |
| C9 | 406.55 | Data | Intermittent, frequently used. |
| C10 | 409.95 | Data | Intermittent, frequently used. |
Note that of the 10 LMR transmit frequencies, the following pairs are 600 KHz apart.
| Frequencies | Comment | Interference Effect |
|---|---|---|
| LMR 1 & 3 | Never a continuous condition. One of the two frequencies is alternatively used as a control channel and the other is frequently used. | Most Frequent offender. |
| LMR 1 & 10 | Never a continuous condition. LMR1 is alternately used as a control channel and LMR10 is frequently busy. | Frequent offender. |
| LMR 4 & 9 | Never a continuous condition. LMR4 is alternatively used as a control channel and LMR9 is used infrequently. | Frequent offender. |
| LMR 5 & 6 | Never a continuous condition. LMR5 and LMR6 are frequently used. | Frequent offender. |
| LMR 6 & 8 | Never a continuous condition. LMR6 is frequently used though LMR8 is rarely used. | Generally a rare combination. |
The arithmetic of the 5 possible combinations of 3rd order mixing products follow:
| Arithmetic combinations that produce 146.34 MHz |
|---|
| 146.34 = 146.94[AR] – (409.35[LMR1] – 408.75[LMR3]) |
| 146.34 = 146.94[AR] – (409.95[LMR10] – 409.35[LMR1]) |
| 146.34 = 146.94[AR] – (407.15[LMR4] – 406.55[LMR9]) |
| 146.34 = 146.94[AR] – (408.55[LMR6] – 407.95[LMR5]) |
| 146.34 = 146.94[AR] – (409.15[LMR8] – 408.55[LMR6]) |
In the table above, AR refers to Amateur Radio, LMR refers to Land Moblie Radio.
The behavior pattern of the intermodulation is observed to be as follows:
| Active control channel | Combinations that generate intermoduation | Interference Effect |
|---|---|---|
| LMR1 | LMR3 (alone) LMR10 (alone) LMR4 + LMR9 (both) LMR5 + LMR6 (both) LMR6 + LMR8 (both - rare combination) | Most |
| LMR2 | LMR1 + LMR3 (both) LMR1 + LMR10 (both) LMR4 + LMR9 (both) LMR5 + LMR6 (both) LMR6 + LMR8 (both - rare combination) | Least |
| LMR3 | LMR1 (alone) LMR1 + LMR10 (both) LMR4 + LMR9 (both) LMR5 + LMR6 (both) LMR6 + LMR8 (both - rare combination) | More than average |
| LMR4 | LMR1 + LMR3 (both) LMR1 + LMR10 (both) LMR9 (alone) LMR5 + LMR6 (both) LMR6 + LMR8 (both - rare combination) | Less than average |
The following two links demonstrate how the LMR signals sound.
The following links demonstrate how the Amateur Radio repeater sounds on 146.94 MHz with and without the protection of CTCSS (PL) tone. CTCSS operation mitigates how badly the 3rd-order intermodulation dominates the repeater operation by holding the repeater in transmit, but is no substitute for the elimination of it. The 3rd order intermodulation signals are so strong (S-15 or better) that when the intermodulation signals are present,the repeater has an effective range of 7 miles or less because the intermodulation signal(s) are generally stronger than most legitimite Amateur Radio signals received beyond this range.
After listening to the previous links one might think that keeping the repeater on CTCSS access is the solution. Unfortunately this is not an acceptable solution. The third order intermoduation signal has been measured to be of S-15 or stronger on a hand held receiver on the IDRL gantry structure. The upshot of this interference is that unless one is within 3 miles of the repeater and running at least 10 Watts ERP, the third order intermodulation will cover the user's signal completely. When none of the LMR transmitters are operating (with the exception of the control channel that operates continuously) Amateur Radio stations of 10 Watts can be heard full quieting up to 30 miles distant.
To eliminate the possibility that the intermodulation is generated inside the Amateur Radio repeater transmitter, receiver, or duplexer, a notch filter from Par Electronics was purchased and installed 1/4 electrical wavelength (at 408 MHz) from the antenna connection to the Sinclair hybrid ring duplexer. The notch filter exhibits a -40 dB reduction in signal (10000/1) between 405-410 MHz. The same notch filter exhibits minor attenuation in the 146 MHz band.
Unfortunately the notch filter produced no observable result in the reduction of the intermodulation signal. The benefit of this experiment however was it proved the intermodulation is not occurring inside the Amateur Radio repeater transmitter, receiver, or duplexer. The downside of this experiment is that (1) It confirmed that the intermodulation signal is being generated externally to the Amateur Radio system and (2) The interference signal is being generated on the repeater receive frequency.
Further experiments with using an adjacent different frequency pair of 146.31/146.91 revealed that the intermodulation is not eliminated or even reduced. This is true because regardless of the output frequency choosen, the intermodulation signal is equal to the repeater output frequency minus the difference of any of five LMR frequency combinations that are 600 kHz apart.
What remains to be done is identifying the specific location on the IDRL gantry structure where the absorption, mixing and re-radiation of the intermoduation signal is occurring on 146.34 MHz. Observations with hand held receivers indicate the intermodulation signal is strong on the IDRL gantry structure, but not sharply well defined from where it is occurring. Patient observations over several hours have revealed that there are at least two hot-spots however.
Further Research via the Internet into other Amateur Radio repeater intermodulation problems suggest a repetitively recurring problem with poor quality coaxial connections, physically damaged and/or internally burned (coaxial braid collinear) antennas, the adjacent presence of rusted chain link fence, rusted link chain, conduits, and/or corroded corrougated tin roofs. Each of these metal structures have the capability for acting as non-linear conductors that absorb, mix and re-radiate new frequency products.
As one may notice from the pictures above there is an abundance of corrosion at this repeater site, especially in the vicinity of the repeater antenna. From some simple observations with hand-held radios it is suspected one of the most likely candidates of mixing and re-radation of intermodulation signals are the two 15' x 10' tin roofs on either end of the IDRL bridge structure. The removal of the two corroded corroguated tin roofs seem to be the next step in the process.
On 28 August 2002 there was a 4 day period of heavy rain. The repeater was temporarily placed back on COR operation. It was noted immediately there was a significant reduction in the strength of the intermodulation interference while the adjacent LMR system remained busy on all or some of 10 channels. It is surmised that the two corroded tin roofs in proximity to the repeater antenna are saturated with water and perhaps are not acting as such good semiconductors on their overlapping edges. This is an important discovery. It raises the question of perhaps artificially re-wetting the two tin roofs when the weather returns to a dry status and determining if the conditions that minimize intermodulation interference to the repeater receiver can be duplicated. One problem remains however, even with the IDRL gantry saturated with water there is another transmitter that is generating a wideband spike on either the start or stop of it's transmit cycle. This broadband spiking was noted on the S-meter of handheld receiver during intermodulation surveys on the IDRL gantry and could not be correlated with the operation of the LMR system. The keying of the repeater is occurring at a rate consistent as if a conversation or rapid paging process is occurring, but no audio is following up after the key-up event. Now it is time to locate this other transmitter with a frequency counter and attempt a transient correlation of the two systems.
On 12 November 2002 the two extremly corroded tin roofs in the vicinity of the repeater antenna were removed. There was no significant reduction in the intermodulation level. The most likely source for the external mixing and reradiation of signals is the 150 foot run of about one dozen or so extremely rusted conduits that are about 15 feet underneath the repeater antenna. Since these conduits cannot be removed or relocated, the repeater antenna must be moved. The current plan is to relocate the repeater antenna to the center span of the gantry to achieve at least 120 feet of separation between the repeater antenna and the large number of corroded conduits that exist on the eastern leg of the gantry.
More infomation will be published here as we tackle this difficult series of problems. I hope this documentation is helpful to other repeater operators who are seeking to tackle similar tough problems like these.
On 24 April 2003 the the repeater antenna was moved 200 feet west to the centerspan of the gantry. All three major problems were solved in one fell swoop.
We finally did it!
| Person | Callsign | Role | Contact |
|---|---|---|---|
| James E. Byrd | WB5POJ | Trustee & Control Operator | 757.864.5961 (ofc) 757.874.3104 (home) |
| John W. Parker | WB4UHC | Control Operator | 757.864.1899 (ofc) 757.898.7147 (home) |
Web page operations: James E. Byrd
Responsible NASA Official: Richard S. Eckman
