How to pick coax adaptors that keep impedance and signal intact
Coax adaptors exist because connector families grew up in different industries and never agreed on a standard. A television cable arrives on an F connector, a laboratory instrument expects BNC, an antenna wants N and a small radio module uses SMA. An adaptor joins two of those mechanically, but it cannot change the characteristic impedance of the cable behind it. That single fact explains why some adaptor chains work perfectly and others produce reflections, loss and a link that fails only at higher frequencies.
Quick Answer
A coax adaptor changes the connector, not the impedance. Keep 75 ohm systems on F, BNC and RCA parts and 50 ohm systems on N, SMA, BNC and SO-239 parts. Short adaptor chains are fine at low frequency; each extra joint adds loss and a mismatch, so use one adaptor rather than three.
| Adaptor | From to To | Impedance | Typical use |
|---|---|---|---|
| F to BNC | TV coax to lab gear | 75 ohm | CCTV, test bench |
| F to RCA | Coax to composite | 75 ohm | Older TV |
| N to SMA | Antenna to radio | 50 ohm | Ham, LTE |
| SO-239 to PL-259 | CB, antenna | 50 ohm | HF radio |
Impedance is the rule that decides everything
A coax line is a transmission line, and its impedance is set by the ratio of inner conductor diameter to dielectric diameter. An adaptor with the wrong geometry introduces a short section of different impedance, which reflects part of the signal. At audio or low frequency this is invisible; at 1 GHz and above it shows up as return loss and a few tenths of a dB.
The practical rule is to keep families together. Television, satellite and CCTV use 75 ohm cable, so F, RCA and 75 ohm BNC parts belong on that side. Radio, cellular and test equipment use 50 ohm, so N, SMA, SO-239 and 50 ohm BNC belong there. A 50 to 75 ohm adaptor is a compromise device, not a converter.
Why a 75 ohm plug on 50 ohm cable still works
The mismatch is small enough that a receiver with margin will not notice, especially below a few hundred megahertz. It becomes a problem when you cascade adaptors or run a weak signal, because each joint adds reflection. Do not build a five piece chain and expect the same result as one correct lead.
The families you will actually meet
F is the threaded 75 ohm connector on every television drop, cheap and not weatherproof unless it has a proper boot. BNC is a bayonet fitting in both 50 and 75 ohm versions, common on CCTV and instruments, and the two versions mate but should not be mixed. N is a larger threaded 50 ohm connector that seals well and handles higher power.
SMA is a small threaded connector used on radios and modules up to several gigahertz, rated for a limited number of mating cycles. SO-239 and PL-259 form the UHF series, an old 50 ohm design still everywhere in amateur radio and CB, with an impedance that drifts above 300 MHz.
Right angle and bulkhead versions
Right angle adaptors save depth behind a panel or a radio, at the cost of a tighter bend radius inside. Bulkhead versions mount through a chassis and give a cable a proper strain-relieved entry. Both are still adaptors, so count them when you budget loss.
Fitting adaptors so they last
Support the joint. An adaptor hanging off a stiff RG6 drop puts all its weight on a soldered centre pin, and that is where intermittent contact begins. Tie the cable within a few inches of the adaptor, or use a short patch lead instead of a rigid stack of parts.
Check the centre pin before mating. An F male pin that is too long bottoms out and bends the female contact; a pin that is too short makes contact only when the cable is held at an angle. Tighten by hand, then a small extra turn with a spanner on N and SMA, never on F.
Weatherproofing an outdoor joint
Wrap self-amalgamating tape over the joint, starting below the connector and finishing above it, so water runs off rather than into the threads. Silicone grease on the threads helps, but do not fill the mating face. A drip loop below the connector does more than any sealant.
Real-World Applications
- Connecting a 75 ohm CCTV camera drop to a monitor that only has BNC inputs, using one F to BNC adaptor per end.
- Taking a television aerial feed into a software defined radio with an SMA input through an F to SMA adaptor.
- Mounting an N bulkhead adaptor through an enclosure so an outdoor antenna lead can enter without strain.
- Joining a PL-259 patch lead to an SO-239 socket on an HF transceiver during a field day setup.
Related Guides
For the full product range, browse our productlist. For a deeper dive, read rg59 vs rg6, and for the next step, see so 239 bulkhead connector.
FAQ
Can I use a 50 ohm adaptor on a 75 ohm TV cable?
It will usually pass a picture, but the mismatch costs a little signal and reflection. For a short indoor run at TV frequencies the effect is small. For satellite or long drops, use a 75 ohm part.
How much loss does each coax adaptor add?
A well-made adaptor adds a small fraction of a decibel below 1 GHz, but a poor one or a long chain of them can add several tenths. Keep the count low rather than chasing premium parts.
Are all BNC connectors 75 ohm?
No. BNC is made in both 50 and 75 ohm versions. They mate physically, which is why the mistake happens, but the 50 ohm version has a different dielectric and pin spacing inside.
Why does my picture break up after adding an adaptor?
The extra joint is probably loose or has a bent centre pin. Reseat it and check that the pin length matches the socket, then bypass the adaptor to confirm whether it is the cause.
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