HF Radio Fundamentals
Another awesome write up from KE4SKY and his generosity in sharing
Why “MF / HF?”
After completing this unit you will be able to explain:
WHAT are the unique characteristics of MF / HF?
WHY does RACES 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…
Edit by Michael- For new HAMs this seems overwhelming. Don’t drown in information. As you learn, this will be a key source of research information. If you cut and paste this onto a word doc, then search each term you can soon be an expert in the 1-30Mhz range of frequencies and be an asset to your team and family:)
Knowledge is far more important than a closet or garage full of gear you don’t understand how to use.
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 shared NTIA channels on 60ms 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
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!
Use of good operating practices!
Line of sight (LOS)
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
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.
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
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.
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!
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: www.morsex.com/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
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
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
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