CB radio signal SWR

Can anyone tell how to fix SWR metter give high reading it over at top of the metter I getting a 4 reading and On the bottom I am getting 1.0 this Is on SWR metter is this to do with my antenna by chance

That reading means a 4 to 1 SWR. Not very good with out a means of lowering or compensating for that ‘4’, or an impedance matching device (tuner or tuned circuit). “Perfect” would be a 1:1. SWR is a ratio between forward and reflected ‘power’ which can be measured by either voltage or current. It’s sort of complicated till you “catch on”… then you wonder why you ever thought it was hard to understand.

A high SWR on channel 40 and low on channel 1 suggests your antenna is too long and needs to be shortened.

Hi I am Get the same reading on ch 40 and CH 1 not it is 1.5 just under it

That’s kind’s odd. But, nothing wrong with a 1.5:1.

In a mobile environment, anything under a 2.0:1 is perfectly adequate.

Just to clarify, since what SWR actually represents is poorly understood mostly -

It’s an indicated measure of the ratio of emitted RF from the transmitter (or linear amp if you’d measuring SWR on the output of that) vs the reflected RF level that tries to return back through the radio’s PA final stage(s) - the higher your antenna mismatch, the worse it gets - feeder issues can cause it as can faulty in-line instruments, even an SWR meter itself can introduce a small mismatch and itself become a source of stray emissions due to how it senses.

Now what causes a mismatch at the antenna ranges from poor/rubbish/inadequate or no grounding (on marconi type antenna or simply unbalanced types that don’t have a resonant fixed matched electrical length counterpoise - on hertzian types, or balanced as they are better known as, such as dipoles and yagi type beams (which is a dipole, paired with a non-electrically connected back reflector element, and many near-resonant director elements that aren’t electrically connected either), these have a self-balanced non-dependency on the actual earth for grounding and the dipole has a resonant earth counterpoise that is physically and electrically a mirror of the driven element, so in practise they are in essence a matched pair of driven elements and a lack of grounding/poor grounding is less of a factor in poor SWR mismatches).

Resonance, the tuned electrical length of the driven element(s) relative to the wavelength you transmit on, has a huge amount to do with SWR results and outside of earth/grounding issues is a big contributor to problems. The established practise to tuning up an antenna is to adjust it so you get a good SWR at the extremes of frequency range you actively use with the centre frequency being at the lowest SWR.

Adjustments typically being a retractable lockable straight driven element on verticals, and possibly where there are loading coils, some adjustment when they are length adjustable (usually by a rotating plastic screw threaded section and lockable). In essence the coil adjustment is the equivalent of altering the ‘divider’ ratio of a pair of inductors/coils by shorting or removing design shorts controlled by the screw threaded rotating adjustment - the ‘Screwdriver’ antenna design (from the ham world) is a good demonstrator of that.

So if you are generally only using a limited group of adjacent or near adjacent frequencies (channel-lised in LMR circle), your tuning centre frequency is either the most middle frequency, where there is no ‘middle’ even division, use either of the nearest to centre frequency but note tuning to the next highest will affect (raise) SWR on the lower frequencies of the group.

With adjustable dual or multi-band antennas, you’ll be repeating the process multiple times, where harmonically related bands will be offset more in resonance each harmonic step by by a starting small offset and getting bigger each step. I mention this because i know there are LMR service users who use both VHF and UHF bands on one set, so will need to get a dual band antenna set up right. Multibanders of three or more get’s complicated as at least one (in a tri-band setup) will be non-resonant completely when the harmonically related band pair are resonant or close in comparable resonance.

It’s not the physical length that defines the resonance, but the electrical length as the electrical length (the actual distance the RF travels and speed of light reduced offset) is what you are actually optimising.

Coax, or the feeder in technical terms, being in good shape and of a good quality matters. Not all coax is equal, and some is definitely not VHF/UHF efficient or low loss. To use a good example, the trashy stuff you get as OEM supplied patch leads for RF antenna passthrough on old world VCR’s, that’s pretty much not fit for purpose in the bigger picture, but is passable for short length. But it can be a good cause for why even at a short length you get seriously poor pass-through on receivers.

So you want a good grade coax as a feeder, and for patch lead interconnects that link up linears to radios, SWR meter and Power meter lashups equally. Not all SWR meters are really fit for purpose at VHF/UHF, especially the trash that was sold into the CB market and dirt cheap temptation buys as a tool. I have a reliable, home-built (but fully certified calibrated, using a professional service spec unit as reference) unit left over from when i was young i still use along with a service grade ex-Pye unit i get recalibrated every few years at a local LMR service centre. Ironically enough, a lot of my pro test gear (not analysers or sig gens) is ex-PYE/Philips Telecom service stuff or age old but stable AVO kit.

I would recommend using either a dual meter (where one indicates output level from the TX, the other gives a parallel ratio of reflection to output power or SWR). Alternatively, but pricer are the single dial crossed needle types where you adjust one needle to show roughly the correct input power or to a calibrated reference point, and the the other (reflected or SWR where switchable, or SWR where not) moves and crosses in sympathy and vertically you read off a calibrated gauge what the SWR is. I favour dual meter or tri-meter types (tri meters show input on one gauge, reflected energy on another and ratio of the two on a third). Tri gauge/meter units are fairly uncommon, so if you find them hard or near impossible to get, a dual is a good average between flexibility and immediate visual check that’s easy to read.

Theoretically, and commonly in practise you read directly from either the output of the transmitter through a patch cable link, or from the output of any linear amp you have in line in your system. It makes sense, to test both post Tx and post Linear mind, as the linear can be an inherent part of a small degree of indicated mismatch.

So ensure you’ve got good coax, or if it’s somewhat aged, ensure it’s not damaged and crushed and if in doubt, replace - for what you paid for it decades ago, you can buy way better grades now for similar money and whilst back in the day there were only a few grades and most were still very HF rated primarily and VHF/UHF and (when SHF was real dark magic territory) SHF rated cable was rarer than hens teeth unless you fancied a weeks wages for the smallest drum of, there’s a lot better availability and scope of choice nowadays. For receivers (all kinds) I use the same low-loss stuff i use on sat receiver setups - technically very out of spec for HF, but on a balance of low-loss vs marginally worse match, it makes little difference in that context.

If you can buy it and justify it by reusing it, there are good antenna analysers out there that you can either directly tune, they emit a low level reference signal, and you can read off the display the resonance and often approximate SWR. Useful for presetting a batch of antennas ready for installation or getting a matched set for clustered or phased arrays. The higher end ones, you can set a frequency sweep range and it’ll tune/emit across the range of the frequency limits and give you a graph representation of resonance at many sample frequencies in the set range.

As for target SWR - simply you are aiming for 1.2-1.3:1 ratio territory, higher 1.x is ok, but when you go into 2.x:1 or higher, the mismatch will affect the PA transistors, and over time the reflected energy excess will compromise the lifespan and performance of the transistors/transistor pack or discrete PA block (the sealed units soldered as one part). I wouldn’t accept and settle for 2.x:1 or higher, maybe 2.0:1 in a temporary setup, but 3:1+ is a no go area, i respect my equipment too much, and given some of it is collectable and very hard to replace affordably if you seriously screw it up (it’s not just PA’s that can get duffed by high levels of reflected energy), i’d rather not be on air for a few days until i can resolve it than put a bad setup to air. I apply the same logic when i (rarely nowadays) do installations for domestic radio users.

All the ‘3:1 is safe’ type myths is just that, a myth and very poor practise to pass on as wisdom. These myths are legacies from when PA ‘blocks’ and modules were valve or hybrid valve based (in hybrids, a transistor is the pre-stage and power final stage is the more electrically robust valve). The valves used were as fragile as china physically, but they could stand some horrendous abuse when it came to extended transmissions, very high duty cycle modes of operation, and still run fine for ridiculous extended periods under SWR mismatches that would trash today’s PA blocks in nanoseconds of worse case mismatch. As far as they were high duty cycle robust and if kept in a static powered state on the LT side and only occasionally powered down on the HT side, very long lived, you could be talking decades of life on some of the higher power RF stage ‘power’ valves. In fact, the emitter in your microwave oven is actually a very specialised type of valve, and very heavy duty versions (and very here means extremely, a country mile of distance difference in heavy duty) still make up the finals in those transmitters used in ground stations and critical uplinks such as NASA use.

Remember, when the soviets used valve tech (and they have some very advanced evolved items that were pretty ■■■■■■ efficient) in their Cold War era ‘EMP resistant’ military electronics, it wasn’t just because they are relatively high survivors vs EMP bursts (they are almost untouched if out of circuit, and hard to damage if they were active - transistors could suffer long term compromised performance even out of circuit and pretty much trashed if active during a burst). It was as much their robust and long lived operation as the EMP resistance which saw some valve elements in their aircraft well into the 90’s, some instances may still be there in miiltary modern designs even.

So, that’s the lowdown on SWR, consequences and significance of mismatches. The whole lot could fill a few encylopedia volumes if you care to research serious technical references.

So, go for low, but don’t bother trying to reduce a 1.15:1 or 1.2:1 to the theoretical ideal of 1.1, if you can get it indicated, you are looking at a then trying to rationalise a probable false postive type mis-indication unless you are one incredibly lucky soul. 1.15 -> 1.2:1 is as low as you ever need go, and your PA’s will have a happy long life.

If you must go on air with such a severe mismatch, keep your overs very short and infrequent as possibly so you don’t give the PA a chance to get RF hot from reflected energy. Likewise, both as it preserves the lifespan of a PA and is general good practise in consideration terms, don’t go using high power output settings where you don’t need to - you’ll be suprised even at 300-400 Mhz territory how much range you can actually get out of 1 or 2 watts with a well setup and optimsed antenna setup.

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