Anatomy of an F barrel (F81)

The humble ‘F barrel’ as it is known, the F81 adapter, is specified by IEC 61169-24:2009. That specification includes an extended performance type good to 3GHz.

It seems that every seller on eBay has worked out that they cannot sell F81 adapters unless they state that they are 3GHz rated… and they almost all lie.


Above is the internals of an F81 purchased as 3GHz rated on eBay. The construction is simply the popular construction used since the 1970s and good to almost 1GHz, good enough for VHF/UHF TV.

Note the construction of the inner conductor connection, it is formed from flat material bent to form a pair of flat springs at each end of the adapter. Material used varies from plated or tinned hard brass / bronze, to stainless or ordinary plated steel.

It is this inner conductor that sets the upper limit for good Return Loss, and extended performance connectors use a cylindrical inner conductor for less disruption of Zo.

The exterior can be anything for low grade zinc die cast or die cast aluminium through plated steel to plated brass.

These connectors depend on sufficient nut torque for good and reliable connection, 3-3.5Nm is recommended. Many low grade cast accessories fail under such torque, stripping thread or more commonly just shearing off.

The market is full of junk, and price is not a good indicator of quality.

Don’t be sucked in by the colour of the insulator.

Above, it didn’t take long for unscrupulous copyists to produce low grade UHF F81 connectors with coloured insulators to trick buyers into thinking they were 3GHz grade connectors. Note the internal contact and compare it with the pic of a real 3GHz adapter later in the article.

Whilst quality connectors are readily available at low cost in the US, they are not here in Australia and since the Americans made the world a safer place, shipping to Australia is exorbitant (~US$60 or A$80 for 100 x F81) and local or Chinese sources are an alternative.

I bought small lots from five different Chinese sellers on eBay and Aliexpress and all were similar to the pic above. The eBay sellers have all refunded my money in full, the Aliexpress seller probably won’t because Aliexpress is by the Chinese for the Chinese… foreign buyers are scammed regularly… a known risk.

In desperation, I searched for the part number of Foxtel’s approved supplier Jonsa’s product, and lo and behold there was a seller in Melbourne and at reasonable prices (~$9/10).

Jonsa APFF/FF3N (Foxtel F30583)


Above, the Foxtel approved Jonsa APFF/FF3N.

Above is a pic of the internals of a 3GHz rated F81 barrel. Note also that the both ends have substantial flat flanges rather than the roll crimp on low grade  connectors. This adapter uses nickel plated brass body and gold plated brass inner conductor. The air spaced section of this adapter (15mm of its 25mm length) calculates to Z0=72Ω, the section with the green insulator will be a little lower… so it is hardly perfect, but a lot better than the <1GHz type.

Above is a pic I have used doing battle with dodgy Ebay and Aliexpress sellers misrepresenting product as 3GHz rated,