Your browser is out-of-date!

Update your browser to view this website correctly. Update my browser now


A Three-Station Multiplex Project

Creative engineering combines three high-power AMs into a single antenna system

Upon retiring in 2000 from my position as chief engineer of KFI (AM 640) and KOST (103.5 FM) in Los Angeles, I thought I was finished with the radio broadcasting business.

Several weeks after retiring, I had lunch with John Price, former studio supervisor at KRLA 1110 in Pasadena, with whom I had worked earlier in my career. Joining us was Dave Berges of ABC Radio Networks.

During lunch, Dave mentioned that he had a friend in Florida, Roy Pressman, who was looking for someone to take care of a radio station in Southern California. Dave asked if it was okay to give my phone number to him. I said okay, thinking the station was probably too far away and I would not be interested. Several days later, Roy Pressman called and told me the station was KBLA on 1580 kHz in Santa Monica/Los Angeles. Knowing the transmitter was an 11-mile drive down the freeway from my residence, I told him I was interested. This is the closest 50 kW AM transmitter site to where I live.

At the time, the station was owned by Radio Unica; several years later, it was acquired by Multicultural Radio Broadcasting Inc. (MRBI). After the ownership change, I continued working part-time, taking care of the KBLA 1580 site. MRBI also owned KYPA 1230 in Los Angeles, with a transmitter site near downtown Los Angeles. The rent for the KYPA transmitter site became so expensive that MRBI decided to move KYPA to the KBLA transmitter site.

KBLA operates with 50 kW using four towers during the day and six towers during the night. The design of the new KYPA system called for two towers; KYPA now shares Towers 2 and 6 with KBLA, using the same two towers for day and night with a power of 1 kW.

In order to share the two towers with KBLA, appropriate filtering was needed. At each of the six towers, filtering was added to prevent the KYPA signal from being fed back to the KBLA transmitter. Filtering was also needed at Towers 2 and 6 to prevent KBLA from feeding power back to the KYPA transmitter. For several years, KBLA and KYPA coexisted nicely on their respective towers.


Several years after the original diplexing operating began at the KBLA site, the owners of KHJ 930 AM decided to sell the valuable land from which KHJ had transmitted for more than 80 years. This sale was partly driven by the fact that KHJ could be triplexed into the KBLA/KYPA system. Subsequently, KHJ did move to the KBLA transmitter site, and now operates with 5 kW day and night using Tower 5 during the day and Towers 3, 4 and 5 at night.

The addition of KHJ required a big increase in the amount of filtering — so much so that the filter cabinets at the base of the towers had to be replaced and larger cabinets installed to accommodate all the components required to prevent each station from feeding back to the other stations transmitter. With all the filters and matching networks installed at KBLA, KYPA and KHJ, the site could be called Coil and Capacitor City!

Ron Rackley of duTreil, Lundin & Rackley Inc. did the design and tuneup of the phasors, tuning networks and filters for all three stations. Kintronic Labs built the filters and matching networks per his design.

Ron’s adjustment method consisted of connecting 50-ohm dummy loads on each of the six outputs of the KBLA phasor and then adjusting the phase and power using a signal generator and vector network analyzer. The outputs were then connected to their respective towers, each of which had been previously adjusted to match the 50-ohm coax cables connecting the towers to their respective outputs on the phasor. After some minor adjustments, KBLA’s day and night patterns were back to normal.

The same procedure was used to make KYPA operational on Towers 2 and 6 and KHJ operational on Towers 3, 4 and 5 of the array.

Ron calculated that with the addition of KHJ 930 5 kW signal, the voltage at the top of Towers 3, 4 and 5 would be in excess of the voltage rating for the guy insulators. Therefore, much larger insulators replaced the original guy insulators near the top of these towers.


The antenna system for all three stations has been very stable since KHJ moved to the KBLA site in 2013. However, on one particular day, the reflected power for KYPA was found to be excessive. The other two stations were still operating with all parameters remaining normal. An inspection of the components in the KYPA matching network and filters did not show any defects.

After a consultation with Ron, it was decided to make some measurements at various places in the KYPA matching networks and filters. Burt Wiener, a well-known consultant in Los Angeles, was called in to make the measurements with his operating impedance bridge. No problems were found. Various measurements were made at Towers 2 and 6 used by KYPA, but still nothing untoward was found.

While making these measurements, it was necessary to keep the stations on the air, of course, and making these measurements was a little like checking your tire pressure while driving down the freeway. After hours of making measurements and finding nothing, we decided to start looking at the towers not used by KYPA.

At Tower 1, we were able to pull a J-plug and isolate the 1230 kHz filter for KYPA, and a simple ohmmeter measurement found the capacitor in the series resonant filter circuit to be shorted. Who would have guessed that a shorted capacitor at a tower used by KBLA and not KYPA would cause reflected power to go high at the KYPA transmitter?

After installing a replacement capacitor and retuning the network, the reflected power was back to its normal low at the KYPA transmitter. This is an example of how mutual coupling between towers can cause a problem not to be where it is expected to be.

As you can imagine, the filtering becomes complex when you consider each of the six towers has to have filtering combinations to keep the RF from the three stations flowing in the right directions and at the same time not flowing in the wrong directions. Towers 2 and 6 have to have matching networks for KYPA, and Towers 3, 4 and 5 have to have matching networks for KHJ, adding even more complexity.


KYPA (formerly KGFJ) had been located on top of a building not far from downtown Los Angeles. Apparently, the owners of the building felt that KYPA could not move and therefore would pay the rent increases. When the rent became too expensive, MRBI decided it was time to move KYPA to the KBLA site, which MRBI already owned. Thus, no more rent to pay for KYPA. MRBI has since sold KYPA and now collects rent from the new owner.

The former owners of KHJ sold the valuable land where KHJ had been located for decades. Therefore, KHJ was also moved to the KBLA site. After the move, KHJ was sold and the new owners are now paying rent to MRBI. This is all working out well for MRBI. The site works well, is stable and gives the three AM stations an economical site from which they can continue to operate.

Marvin Collins started working in radio in 1954 at KCBH FM 98.7, Beverly Hills. The next eight years, he was employed at KPOL AM 1540 and KPOL FM 93.9. From 1965 until 1976, he worked at KRLA 1110 AM in Pasadena, Calif. In 1976, he moved from KRLA to KFI where he became chief engineer in 1980. He remained at KFI until his retirement, late in the year 2000.