RT-524 Frequency Range and FCC Limitations

[ODRotorHead]

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This has probably been covered before, but . . .

In the process of collecting all the "bits" to install an RT-524/VRC (AN/VRC-46) in my MUTT, I asked a lot of questions about operating the radio and got lots of different answers. In order to learn more, and to stay "legal," I went ahead and took the test to qualify for an FCC license. Even after that, I still didn't (don't) fully understand the radio spectrum and how our old military radios fit into it.

I suspect that I'm not the only one with questions and a degree of uncertainty. This is all still new to me, so if you're an experienced Ham, please chime in and correct the record.

I learn better from pictures than from text, so I cobbled together a graphic that might help (click on the image for a larger version):
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RT-524 Spectrum + 6 Meter 002.jpg

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The main graph is the FCC allocation of the radio spectrum the RT-524 can operate on - 30.0 MHz to 75.95 MHz. The left side, 30.0 MHz, is at the upper edge of HF. Moving to the right, from 30.0 MHz up to 75.95 MHz, is the lowest portion of the VHF segment. As you can see, most of that real estate is already occupied. You can listen on any frequency all-day-long, but transmitting on unauthorized frequencies may constitute illegal interference.

Steve Haney (haneyelectronic.com) explained it this way: "Radio frequencies are like real estate. Everything belongs to someone. It may be private property, business property, or government property but it all belongs to someone."

Near the center of the graph is a section from 50.0 MHz to 54.0 MHz, labeled "Amateur Radio." That part of the spectrum has been set aside for use by anyone with an FCC License (Technician, General, or Amateur Extra*). Hams alternately refer to this segment of the spectrum as the 50 MHz, or 6 meter band. The expanded graph at the top illustrates the types of activities typically allowed on the 6 meter band. The term "phone" refers to voice communications.

The 6 meter band has some interesting attributes and hams sometimes referred to it as "The Magic Band." "Propagation shapes every aspect of the magic band. Local communication often works through direct VHF paths, just like on 2 meters. But when ionospheric layers cooperate, they bend or reflect signals far beyond the horizon. Sporadic E forms quickly and unpredictably. With tropospheric ducting, operators near coastal areas sometimes enjoy extended HF-range contacts. Furthermore, meteor scatter can briefly enhance the band. During meteor showers, short bursts of reflection help signals travel hundreds of kilometers."

To add one more level of complexity, the Ham community has established a voluntary structure called a Band Plan which sets aside specific frequencies for specific purposes. There's a National Band Plan and there are Regional Band Plans. If you'd like to know more about the 6 meter Band Plan program, refer to https://www.arrl.org/band-plan for more details.

If you have a single RT unit you'll be operating Simplex - receiving and transmitting on a single frequency. If you have a second radio, or an aux receiver, you can operate Duplex - receiving and transmitting on different frequencies. Duplex allows operation with repeater stations and expands the number of frequencies you can use.

One additional consideration, is that modern radios tune in much smaller increments (infinite with a VFO) while the RT-524 tunes in 50 kHz steps. Many of the frequencies listed in the Band Plan simply aren't tunable on old military radios. For example, the 6 meter calling frequency is 52.525. Unfortunately, the closest we can get to that is 52.50 and 52.55.

Taking all of that into account, listed in the upper left of the illustration are the four Simplex frequencies in the 6 meter band that are available to us. If you have a two-radio setup, or an aux receiver, there are additional repeater frequencies available.
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Antenna Warning 001.jpg

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Antenna Warning 002.jpg

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Maybe someone with more knowledge and experience can add information about antenna/RF safety, radiation patterns (vertical vs. horizontal antenna orientation), ground planes, etc.
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Monopole Radiation Pattern 001.jpg

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*License categories such as Novice and Advanced are still valid, but are no longer issued.
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[FoxMike]

The military radio guys typically operate on 51.0 mhz.
Any other 6m FM simplex frequency is generally fine too and it is a very underutilized band.

For the most part, aside from a few area in the US, the lowband (what the 524 covers) spectrum is either no longer or very sparsely used. The US mil does use parts of the lowband, although a secure frequency hopping mode is more commonly used.

As far as antenna configuration, generally you want yours to match who you're talking to. Ie vertical to vertical. For RF safety, don't touch the antenna ( or metal next to it) while transmitting and you'll be fine. The frequency range and power levels are low enough that you should be below the RF exposure limits. There are websites to calculate what this are should you desire.

These Radios are most fun if you have another local ham friend or military vehicle guy in town to talk to. Otherwise you'll be listening to static and talking to yourself.

[Horst]

great for you guys in the US that you can legally use the military radios in the 6m band. Here in Europe, the 6m band is not authorized to be used, so my ham radio license is of no help.

On the other hand, the few times I have switched on the radio, I had never heard any signal.

But maybe my radio is dead by now, it did work at some point in time but these old radios do die over time. Capacitors go bad and so do the tubes.

[ODRotorHead]

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Horst

I've been using a Wouxun KG-Q10H handheld to communicate with "home" when I'm out on the road with my RT-524. The KG-Q10H is four band capable - 6m, 2m, 1.25m, and 70cm. It also has aviation VHF - receive only.
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[ODRotorHead]

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A lot of guys have mentioned a loss of range when transmitting with the antenna tied down.

Ignoring ground plane and other factors, this is a simplified illustration of an AS-1729 radiation pattern with the antenna in both positions:
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Simplified Radiation Patterns 001.jpg

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[FoxMike]

That is a good visual. In addition to a lot of the RF energy being directed downward, you lose signal due to cross polarization. I seem to remember reading that the whip antennas are actually dipoles? For being in a convoy, I doubt the configuration matters too much due to close proximity.
I dug out the 1981 version TM 11-5820-401-10-1 for the radio sets. This version is neat because it was the M151 in the pictures. Found another online, but it has the HMMWV instead. Attaching a few interesting pictures from that.

ETA: I was able to get an acceptable SWR on the 10m ham band out of the whip antenna. So using the tuner on the ham rig, I made some HF contacts while sitting in the back seat of the M151. That was neat.

Screenshot 2025-09-09 141231.png

Screenshot 2025-09-09 141354.png

Screenshot 2025-09-09 141422.png

[ODRotorHead]

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Thanks for the additional info. The following might answer your question (monopole vs. dipole):

“The AS-1729/VRC is an omnidirectional, vertically polarized whip antenna assembly that provides transmission and reception of radio signals (very high frequency (VHF)) between 30 and 76 MHz.” TM 11-5985-262-15

DESIGN: The antenna consists of two mast sections, the upper AS-1095 and the lower AS-1730 connected to the MX-6707 Matching Unit/Base assembly. The 2 fiberglass mast sections are center-fed in a 10' high, vertical dipole configuration.

Note that this antenna is NOT a base fed, “quarter-wave” vertical that would ordinarily require a ground plane to control the base impedance and to reduce ground losses. It’s a different animal. As a center-fed vertical it ideally does not need a ground plane.

The center-fed design works by keeping the highest RF currents (that cause the actual radiating) further up the whip near the center and further away from vehicle/installation obstructions near the base. This, versus the max current/radiation that actually occurs in the spring (!) area of a base fed vehicle antenna closer to the lossy ground and buried next to a pattern-disturbing fenders and other metal parts. It’s a very good design.

The whip center feed point is driven from a choke balun residing in the upper part of the MX-6706 matching unit.

One output lead of the balun is connected to the upper whip section. The other balun output lead is connected to the upper end of the lower, tubular whip section via the Whip-Spring connector assembly and internal conductor.

Not my work, but it's a good explanation.
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[rickf]

I thought I read in the manual that one of the bands was in the 100 watt range.