Marlin Firearms Forum banner
1 - 9 of 9 Posts

2,188 Posts
Discussion Starter · #1 ·
This is the student guide from training provided for Auxiliary Communications Service volunteers who have recently upgraded from an FCC Technician to General Glass Amateur Radio License:

MF / HF Radio Fundamentals

Why “MF / HF?”

After completing this unit you will be able to explain:

WHAT are the unique characteristics of MF / HF?
WHY does ACS use them?
WHICH MF / HF bands are used for EmComm?
WHEN are MF / HF preferable to VHF / UHF?
WHY is choosing the right frequency so important?
HOW is EmComm different from working Dx?
BASICS of equipment setup and antennas
OPERATING hints and practices...

What is “MF” / “HF”

Versus UHF and VHF

The entire MF / HF bandwidth is only 29.7 MHz., but it does amazing things Beyond Line Of Sight which are impossible at higher frequencies!

Band Frequency Range Amateur Band
MF 0.3 - 3 MHz 160 meters
HF 3.0 - 30 MHz 80, 75, 60* 40, 30, 20,
17, 15, 12, 10 meters
VHF 30 - 300 MHz 6, 2, 1.25 meters
UHF 300 - 3000 MHz 70 cm, 23 cm and up

Use the shared NTIA channels on 60m when conditions preclude reliable use of either 40 or 75 meters.

WHY use MF / HF for EmComm?

Reliable “24 / 7“ Communication!

Not dependent on repeater infrastructure!
24 hrs / day, 7 days / week, year-round

Wide-area coverage
Statewide / Regional / Nationwide

Ability to change bands, modes
Work around problems or conditions

How do MF/ HF work?

Requirements for success:

Knowledge of equipment, procedures and techniques that “work”!
Knowledge of propagation!
Proper frequency selection!
Suitable antenna design!

Reliable equipment!

Use of good operating practices!

Propagation Modes:

Line of sight (LOS)
Ground wave
Classical sky wave
High angle (NVIS)*
*Near Vertical Incidence Sky wave

Line of Sight (LOS)

Direct wave mode
Most common at VHF, UHF and above
Limited by terrain absorption

Path blockage
At MF / HF frequencies generally limited to stations “within sight” of each other.

Ground-reflected wave mode
Reaches Rx station after ground reflection
Arrives later than direct wave
Can cancel or enhance direct wave depending upon frequency, reflection
Generally weakens direct wave reception.
Space wave mode
Vector sum of direct + ground reflected wave
Use of “high ground” reduces terrain effects
Good LOS propagation for short range paths.

Ground Wave Mode

Vector sum of space wave + surface wave
Useful to 50 miles in “ideal” conditions
Affected by terrain, vegetation, atmospherics.

Surface wave mode = Component of ground wave traveling along the earth’s surface
Affected by ground conductivity
Direct and ground reflected waves tend to cancel when antennas close to ground
Signal diminishes with antenna height
Not useful more than 1l above ground
Less attenuated with vertical polarization.

Classical Sky Wave

Uses ionospheric reflection
Unique to MF and HF!
Enables long range communication!
Proper frequency selection important

Shorter the distance, lower the MUF.

“Skip zone”

Beyond ground wave
“Too short” for “normal” sky wave
Whip antennas poor performers on short paths**

**for NVIS on 75, 60, 40m bands turn mobile whip horizontal,
use antenna coupler, attach jumper cables to electrically bond vehicle
frame to a fence wire or highway guard rail if operating fixed.

Near Vertical Incidence Sky Wave ( NVIS )

Vertical radio energy radiated at a low enough frequency is reflected back to earth at all angles.
Effect similar to taking a fire hose with a “fog”nozzle and pointing it straight up!

Omni-directional pattern without “dead” spots.

Continuous circular radiation pattern.

Proper frequency selection is critical!

MUF lower at higher angles of incidence.

Signal strength doesn’t vary with small changes in operating location or height

Less affected by terrain, vegetation

Efficient for short paths if a proper antenna is used.
Generally requires takeoff angle >70 degrees

Proper antenna necessary to suppress ground wave amplitude to prevent fading

1/2 wave dipole at 1/4 to 1/10 l above ground to cause radiated energy to be directed vertically

Antenna is then Omni-directional and broadside orientation to receiving direction is unnecessary!

“There is no ‘skip zone,’ unless you create it!” Summary of FM24-18, Appendix “N”

Understand the relationship between angle of radiation and the effective operating distance

Use a dipole at 1/4 to 1/10l above ground

Physical height is not critical, +/- 0.1l is OK

Elevate 25 ft on 75-80 and 40m if you can do so safely

If you can’t erect a wire antenna safely, lay insulated wire on the ground! - Use transmatch and it will work

Permits 400 mile, “24 / 7” ops w/o “skip”

NVIS is best when:

Operating area not conducive to ground wave
Mountainous terrain

Stations on the net are located in “skip zones”
Mobile whips or “usual” operating techniques.

Areas of high signal attenuation
Wet weather conditions in heavily forested areas

Operating positions below surrounding terrain

Wide-area SAR and disaster relief operations
Helicopters or light aircraft close to the ground.

How Emergency Communications are Different from DX?

EmComm usually requires SHORT paths!

Shorter path, needs higher radiation angle

Higher angle means a lower MUF

Means using 75-80 meters at night

And 40 meters during the day

Coverage within the active NVIS radius is usually quite “even.”

Critical frequency for F-layer propagation (fo) depends on Solar Flux Index (SFI)

Often below 4 MHz at night

Rarely above 6 MHz in daylight

Fluctuates on 11 year cycle

For current SFI listen to WWV every three hours starting 0000 UTC, 18 min. past hour

Maximum Useable Frequency - MUF
Versus Frequency of Optimum Traffic (FOT)

MUF related to critical frequency (fo) by takeoff angle: MUF = fo /Sin(takeoff angle)

Because of high variability in ionosphere operating near the MUF is unreliable

FOT is the highest frequency where ionosphere propagation is 90% reliable
For reliability operate 20-25% below MUF

As SFI increases, MUF increases

For NVIS winter is more critical than summer

In low SFI 75-80 meters is useful in daylight
But not at night...
Winter 75-80 meters may go “long” late at night

Momentary outages occur on daylight paths during solar flares...

Evening QSB (fading) during geostorms.

Greatest challenge in low SFI is short path!

Under 300 miles predicted MUF always above 2 MHz, but FREQUENTLY not above 3 MHz

Limitations of amateur bands...
40 meters unusable for EmComm in low SFI

Locations within 300 miles in “skip” zone.

Medium Frequency (MF)Use:

Why 160 meters?:

Useful for all paths at night

Only “MF” band that amateurs have

May be the only way to make NVIS useable in some instances.


NOISY !#$%@^*&!!

Ionospheric absorption is much greater

Harder to radiate a strong signal

Antennas are larger

Resistive losses must be minimized.

Characteristics of MF not always in our favor because:

Low antenna gain

Lower antenna efficiency

Higher path losses

Higher noise floor

Requires greater RF output power to establish and maintain reliable contacts.

Characteristics of the 160 Meter Band

During daylight 160 meter sky waves almost completely absorbed in the D-layer

Atmospheric noise of much higher intensity
- Expect 5-8 dB increase from 40 to 80 meters
Expect 8-9 dB increase from 80 to 160 meters!

Antenna Fundamentals

Wire antennas
Home brew versus “bought”
Fixed versus portable and expedient

Temporary - versus permanent supports
Or NO supports
( lay insulated wire directly onto ground, use coupler / transmatch)

Wire Antenna Formulas

LOOP (feet) = 984 / MHz

DIPOLE (feet) = 486 / MHz

VERTICAL (feet) = 234 / MHz

Dipole, cut to working frequency is a better performer

Last third of dipole ends can be folded back 90 degrees, saving space without significant reduction in gain

Hints on homebrew wire antennas: This Old Dipole

Getting your support lines up in the air
Slingshot, etc. small line first
Nowhere in proximity to power lines!!

Remember that trees move!
Support ropes, pulleys and counterweights

Insulated wire dipoles
Require NO elevated supports
Will “work” w/transmatch when laid on ground!
Proximity to ground or structures causes discrepancy in calculated vs. resonant length
Test the antenna in the same environment in which will be deployed
Transmatch or cutting to working frequency highly recommended.

Insulated wire desirable for safety and durability.
Commonly available, at low cost
Less affected by elements, less risk of RF burns
Physical dimensions are about 5% shorter due to decreased velocity of propagation
Works laid on the ground when used with transmatch (necessary in expedient situations).
Wire - Stranded 12-14 AWG, SAE type oil resistant, insulated
Hard-drawn, Copperweld, or insulated
Insulators, ceramic, plastic, glass, improvised
Feed line, ladderline, coax
Connectors, N, UHF

Must use a current mode balun with coax-fed dipoles

Ferrite beads in-line on coax
Adds weight to antenna, use center support at feed point
Place close to the center feed point
Fair Rite #2643102002, fits up to RG-8
Five beads per antenna, for 15-20 µH total common mode inductance
Use heat shrink, or tape to wrap beads
Prevent breakage or movement
Or encase in PVC filled with DAP
non-expanding foam, with SO-239s
on short length of RG213
On AC / DC power leads to minimize stray RF pickup
If antenna close to rig or other electronic devices
Noise reduction in power supplies, etc.

Learn How to Solder…Install PL-259s the right way

Avoid too much heat, damages dielectric

“Solder It” Kit - low temp paste really works!
Pre-tin cheap nickel plated connectors

Recommend silver-plated Amphenol instead!

Better yet, avoid PL-259s altogether, use “N”
PL-259s are not constant impedance
PL-259s are not waterproof.

Mobile Operation Considerations

Noise reduction - find source, use ferrite choke

RF and DC grounding, bonding of body panels

Power connections, vibration and water proof

Low voltage disconnect

Mobile antennas

Vertically polarized whips poor on short path

Lean / bend long whips horizontal

Use a transmatch

Dual -Hamstick-dipole adapter - if stationary

Other mobile / portable antennas that “work”

For NVIS and EmComm applications

Basic Operating Skills – Proper Use of Transceiver controls

“Clarifier” / RIT, notch/ shift, RF gain, AGC, attenuation

Power supplies, batteries and charging

Equipment installations, wire gages and connections, fuses, diodes

Fail to battery and reverse polarity protection

Minimum field test equipment, tools

Lightning protection...

Where to put the equipment…
Close to the battery and within 100 ft. of antenna
Ergonomic operating position, writing area
Good air flow around equipment
Ventilation of battery banks

Equipment “nuts & bolts”
Importance of RF ground,
Discussion of “hot chassis” problems
Antenna tuners
Which rigs?

Operating Practice - Minimum transmit power to maintain reliable communications

Reduce power when conditions are favorable!
5-25w enough on SSB with a full-sized dipole!
25-50w is usually needed for vertical mobile whips.
Reducing power permits batteries to last longer
But in noisy conditions even 100w may be marginal

“Commanding the frequency”
Dealing with malicious interference

Links for More Info
This Old Dipole
New Page 1
SHAred RESources (SHARES) High Frequency(HF) Radio Program | Homeland Security
HFLINK | ALE HF Automatic Link Establishment HF Interoperability HF LINK
GAREC - International Amateur Radio Union
Winlink Global Radio Email
W4WVP- The Arlington Amateur Radio Club » About MT63 Digital Messaging
WB8NUT - Digital Modes Information Page

Premium Member
2,618 Posts
Studying for my exam.... My wife's already got her general license

28 Posts
I've been teaching some of the locals that have opened their eyes, Its a great feeling when they pass their Tech

Super Moderator
12,057 Posts
I suck at morse. I know they dropped it from getting a tag but it always seemed that in a SHTF it would be the most useful thing to know from a communications standpoint since QRP rigs can work off flea power.
That said Baofeng makes some danged attractive 2 meters rigs for ridiculously cheap prices..

505 Posts
We had HF radios in our aircraft when I was in Africa, we use then to communicate to our base camps.
I know one thing you don't want to be holding unto the end of the antennae wire when some body keys the mike.
Burns like hell, don't ask me how I know:vollkommenauf:

23,796 Posts
Morse is a really good thing especially if you rig up an arc generator in a emergency.

If you have a hard copy you can write your message in code then transmit.

2,898 Posts
wa0rox, cliff in augusta, ks. I am in a group that teaches for the tests. I am also an extra since 95 (when code was necessary at 20 wpm) I am also a w5yi voulenteer examiner here in butler county ks. ham radio is a great hobby, there are so many different things to do, its hard to burn out. ever talked to the space station? its easy when you finaly get around to it.
1 - 9 of 9 Posts