The Hamfest Hardline Treasure: 75-Ohm Hardline

The Hamfest Hardline Treasure: 75-Ohm Hardline

Don't Walk Past That 75-Ohm Hardline at the Hamfest

One of my favorite places to browse at any hamfest isn't the tables covered with shiny new radios—it's the pile of dusty commercial surplus tucked underneath them.

If you've attended enough hamfests, you've probably seen coils of Andrew/CommScope, CATV trunk cable, or other commercial hardline with a handwritten tag that simply says:

"75 Ohm Hardline"

More often than not, it sits there until the end of the show. And then you get it for cheap!

Many amateur operators immediately dismiss it because they've heard all their lives that "ham radio uses 50-ohm coax."

That assumption often leaves some of the best transmission line in the building waiting for someone who understands a little transmission-line theory.

For many amateur installations, especially long runs on VHF and UHF, quality surplus 75-ohm hardline can outperform ordinary 50-ohm flexible coax simply because its attenuation is dramatically lower.

Why 50 Ohms?

The choice of 50 ohms was never because it was the perfect impedance.

Engineers discovered many decades ago that:

  • Around 30 ohms provides maximum power handling.

  • Around 77 ohms provides minimum attenuation.

The RF industry eventually standardized on approximately 50 ohms as a compromise between those two characteristics.

Cable television engineers had a different objective.

They wanted the lowest possible transmission loss over many miles of cable, so they standardized on approximately 75 ohms, which is very close to the theoretical minimum-loss impedance for air-dielectric coaxial cable.

Neither standard is "better."

They simply optimize different engineering goals.

The Mismatch Isn't Nearly as Serious as Many Think

A 50-ohm transmitter connected to 75-ohm transmission line does create an impedance mismatch.

At a single junction, the reflection coefficient is:

Γ = (75 − 50) / (75 + 50)

= 25 / 125

= 0.20

The corresponding standing-wave ratio at that junction is:

1.5 : 1

Only about 4% of the incident power is reflected at that discontinuity.

The mismatch loss is only about:

0.18 dB

That is an extremely small loss.

However, it is important to understand that a real feedline has two transitions:

  • 50 Ω transmitter → 75 Ω feedline

  • 75 Ω feedline → 50 Ω antenna

Because a transmission line transforms impedance according to its electrical length, the transmitter does not always see exactly 1.5:1 SWR.

Depending on the electrical length of the cable, the transmitter-end SWR may vary. In practical installations, measuring the completed system with an antenna analyzer is the best approach.

Where the Real Loss Comes From

Most amateur stations lose far more RF in coax attenuation than in impedance mismatch.

A few tenths of a decibel from mismatch usually have very little effect.

Several decibels of cable attenuation absolutely do.

That is where quality hardline shines.

A Real-World 70 cm Example

Let's compare a typical 100-foot feedline at 440 MHz.

Example 1 – Conventional Full-Size RG-8

Typical attenuation near 440 MHz:

Approximately 4 dB per 100 feet

Starting with a 100-watt transmitter:

Power reaching the antenna:

Approximately 40 watts

Roughly 60 watts has been lost as heat in the coax.

Example 2 – Andrew/CommScope P3 75-Ohm Hardline

A common surplus cable found at hamfests is Andrew/CommScope P3 CATV trunk cable.

Typical attenuation:

Approximately 1.6 dB per 100 feet at 440 MHz

With a 100-watt transmitter:

Power arriving at the antenna before considering mismatch effects:

Approximately 69 watts

Even after allowing for the additional loss associated with the 50-to-75-ohm impedance mismatch and connector transitions, the hardline can still deliver substantially more RF to the antenna than the RG-8.

In this example:

  • RG-8: approximately 40 watts

  • Andrew P3 Hardline: approximately 60–69 watts, depending on the completed installation

That is a very significant improvement.

The receive side benefits just as much because feedline loss works both directions.

Every decibel saved improves weak-signal reception.

Why 75 Ohm CATV Hardline Performs So Well

Many people think hardline is simply "big coax."

It isn't.

The construction is quite different.

A typical Andrew/CommScope 75-ohm CATV hardline consists of:

  • Black UV-resistant polyethylene outer jacket

  • Corrugated aluminum outer conductor (which is also the RF shield)

  • Gas-injected foam polyethylene dielectric

  • Copper-clad aluminum center conductor

Unlike RG-8, there is no braided shield.

The corrugated  tube serves as the shield and the outer conductor.

This provides:

  • Extremely low RF resistance

  • Excellent shielding

  • Outstanding mechanical strength

  • Very low attenuation

  • Low or no PIM issues if jacketed (for repeater folks)


Why So Much Good Hardline Ends Up at Hamfests

Many hams wonder why commercial CATV cable is removed if it is still perfectly usable.

The answer is that commercial cable systems upgrade for many reasons unrelated to cable quality.

Examples include:

  • DOCSIS upgrades

  • Fiber node splits

  • System expansion

  • Capacity upgrades

  • Storm damage

  • Road construction

  • Utility relocation

  • Network redesign

Entire sections of excellent hardline may be removed simply because the system configuration changed.

Much of that cable eventually finds its way into the surplus market, where hams can often buy it for pennies on the dollar.

Excellent Uses for 75-Ohm Hardline

Quality surplus hardline is an excellent choice for:

  • HF base stations

  • VHF stations

  • UHF stations

  • Repeaters

  • Scanner antennas

  • SDR receivers

  • ADS-B receivers

  • Amateur television

  • Long tower feedlines

  • Remote receive sites

  • Beverage antennas

  • Satellite receiving systems

Whenever feedline attenuation is important, quality hardline deserves consideration.

When Impedance Really Matters

There are applications where substituting 75-ohm cable is not appropriate.

Examples include:

  • Quarter-wave impedance transformers

  • Precision phasing harnesses

  • Network analyzer calibration

  • Laboratory measurements

  • Precision microwave systems

  • Carefully engineered filter networks

  • Some amplifier output networks

  • Commercial broadcast transmission systems

These systems intentionally depend upon a specific transmission-line impedance.

Inspect Before You Buy

Like any surplus equipment, inspect the cable carefully.

Look for:

  • Crushed sections

  • Sharp bends

  • Water intrusion

  • Jacket damage

  • Corrosion

  • Missing connectors

  • Unknown manufacturer

  • Unknown part number

If possible, identify the exact cable model and look up its published attenuation before purchasing.

Don't Forget the Connectors

The connectors often determine whether surplus hardline becomes a bargain or a headache.

Commercial CATV cable may use:

  • Proprietary compression connectors

  • CATV pin connectors

  • F connectors

  • Type N connectors

  • Various adapter systems

Fortunately, many connector kits remain available, and adapters can often be fabricated or purchased. 

Even after buying connectors, surplus hardline is frequently much less expensive than purchasing equivalent new low-loss 50-ohm cable.

Final Thoughts

Amateur radio has always rewarded operators who understand the engineering instead of simply repeating rules of thumb.

The statement:

"Never use 75-ohm coax on amateur radio."

is simply too broad.

A more accurate statement would be:

In many amateur installations, especially long VHF and UHF feedlines, the reduction in attenuation provided by quality 75-ohm hardline can more than offset the relatively small performance penalty associated with the impedance mismatch.

That does not mean impedance doesn't matter.

It means attenuation also matters—and often matters more.

So the next time you're walking through a hamfest and see a dusty roll of Andrew or CommScope CATV hardline sitting under a table with a bargain price, don't dismiss it just because it says 75 OHMS.

It might just be one of the best bargains in the entire building.

As with so many things in amateur radio, understanding the engineering behind the equipment often leads to better decisions—and occasionally to walking away with several hundred feet of professional-grade transmission line for less than the price of a short roll of new consumer coax.

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