Is my antenna too big for my unit?

Hi, All!

I am new to the forum and brand new to the BC world. I bought my first CB about a month ago for off roading, mostly for just communicating with my party and/or as a communication source for emergencies while in the back woods. I had purchased a Midland 75-822. It’s the one that is modular, that can be mounted in your vehicle or used as a hand held. I had bought a cheap magnet mount antenna that I wasn’t happy with. So I recently received a Tram 3700 trucker CB antenna. I was about to install and tune it with an Astatic compact SWR meter…but I got cold feet. I all of a sudden thought, “Whoa, hold up! Is this too much antenna for my unit?

So, long story short: Will a Tram 3700 work for this CB? Or should I pick up a larger CB head unit?
My brother in-law mentioned that you can get accessories that can amplify your signal, but does this model already have the ability for a strong enough signal? I have researched online but have found nothing yet. So, if you guys have any thoughts, or any info of any kind, it would be much appreciated.

Thank you,
GVBW

To put a few things in perspective - there’s really no such thing as an antenna to much for a rig. Provided it’s eventually terminated to the correct manner (usually 50 ohm nominal impedance, unbalanced, to usually whichever RF grade socket is used on the rig (PL259 plug matching to the rig’s SO239 socket as was pretty much that standard)).

Where it can matter is more a technical/legal compliancy based issue.

If the EIRP (on vertical antenna setups) or ERP (effective output level) is both safe and meets the license mandated limits or is within them, you’re good to go. If not, you may need to use a lower output setting on the rig along with potentially an inline attenuator - but on most factory state rigs I’ve dealt with in the past in the UK (4 Watts PA output max), there’s rarely an issue with anything non-extreme and even with an efficient minimal antenna (a half-wave type where it has typically unity gain characteristics) then worst case scenario is reducing output level at TX. Here in the UK, there was a 2W ERP limit and the max 4W TX power limit was based on the then mandatory use of a quarter wave vertical antenna which was dB lossy hence the imbalance between output and ERP.

So as long as, rig safety wise, you can get an SWR around 1.5:1 or better then you’ll be doing no added damage long term to the PA final of the rig. Once you try to long term operate with a setup in the 2:1 SWR or higher territory out of necessity (shouldn’t be the case) then keep transmissions short and subsequently reduce the impact of the mismatch.

In mentioning keeping transmissions short, aside from general good practise, most ‘domestic’ grade radio kit (including CB stuff) isn’t built with great hulking heat dissipating heatsinks on the PA transistors and definitely was never designed for ‘long overs’ as we describe extended length transmissions in the ham radio world.

Not that it should be of significance in a vehicle where antenna is external mounted, but if you find the mic feels statically ‘hot’ (tingling sensation to your operating hand) or you get headaches occur not due to audio noise or usually harsh overdriven transmitted audio from many stations, then you may be getting undue levels of RF leaking back into the vehicle - this can be induced by the field of the transmission or directly back as reflected energy via coaxial feeder due to poor SWR.

All that aside, if it’s credibly sound construction and setup correctly/adequately, and hopefully license complaint where it matters, you should be good to go.

What I will, however, mention is where an antenna has notable gain characteristics above unity level (not specifically the one you mention which isn’t an item I’ve seen) then you may find your receiver performance suffers rather than seem to get better reception of more distant or quieter signals.

If the rig has what was commonly factory fitted back in the day, the dreaded 255khz ceramic filter (as found in most hi-fi tuners on their FM receive sections), then you can expect lots of RF probably affect your radio even using a quarter wave antenna as with that filter, receiver characteristics are dire (to put it bluntly) on a receiver circuit which was probably substandard to start with. Add in an antenna with actually gain (unity gain isn’t going to worsen it by much) and whatever interference you get on a common quarter wave from adjacent frequencies/channels and out of band sourced harmonics is really going to start getting unbearable.

So I’d say before you do anything, determine if the rig has a ceramic or crystal filter. If it’s ceramic, replace it with an 8Khz or 10Khz crystal filter - it’s a workshop job, really, done right as some ancillary components may be altering to ‘center’ the filter window as there’s an impedance mismatch that’ll occur that you’ll need to reduce/remove. Then you’ll find your stock antenna setup receiver performance improves considerably (notably where your local has a lot of local activity on 27Mhz). Then you’re in a position where a better positive gain antenna based receive gain can actually make a usable difference to what you hear.

Where a crystal filter makes a huge difference receiver wise, is the fact it’s cutting off sharply most properly modulated adjacent frequency transmissions as a 10khz item passes about 5 kHz each side of centre frequency and center frequency (the one you dial in or is preset where channelised as in CB), where an 8Khz item passes centre and about 4khz each side of. Either are actually suitable for 10khz spaced channelised use as with most 27mhz CB, but where traffic is overdriven and over modulated (typical CB as I recall here), then audio heard will be tight and possibly distorted due to bad setup other end so then 10khz is better and more favoured.
On a typical 255khz ceramic item, the receiver is hearing center frequency and about 125khz each side, so there’s no mystery to why you get interference from adjacent and at least a few adjacents to the parallel adjacent frequencies/channels. Add in the shallow response of a typical broadcast tuner ceramic filter in and it’s ■■■■ if you’re used to decent receivers.

On my set back in the day, I had a switched 8/10khz xtal pair so could use whichever suited the situation, but that’s not exactly a plug n play mod - in fact, any filter swap isn’t a plug and play mod as due to filter tolerances you certainly need a Sig gen and be fit to swap out ancillary LC circuit components because of needing to properly center the filter window.

One consequence of a crystal filter, insertion loss aside and needing more setup, is it’ll make you more aware of how much off frequency transmissions are out there due to mostly idiots who tweak with zero idea thinking more output = better and tweak everything in sight until they stumble on the PA circuit eventually. But from experience, a xtal filter as a contribution to improved reception far out ways the additional work to properly fit it.

Ceramics replacement is easier provided the replacement had the same insertion loss and impedance characteristics which replacement items for where normally used are pretty standard and characteristics consistent, not that I can ever envision someone bothering when they could replace a duff ceramic with a crystal item.

But the double edge of high gain antennas means even with a moderate reciver adjustment (crystal filter), there’s a much higher chance of a swamped receiver when you get a lot of high output local-ish activity - but the problem is far less crazy than you’d experience with a ceramic filter.

Some rigs are notoriously bad from the outset due to cruddy (even by CB standards) cheap-■■■ design (taking a mostly sound reference design and par it to bare minimum cheap to manufacture levels). Original CYBERNET (and licensed design used) chassis types weren’t bad and not outstanding (by CV standards) being fairly sensitive and at least having some notable receiver selectivity characteristics, Uniden and licensed use example chassis were ’quieter’ due to better receiver selectivity which meant that typically you actually got to hear the quiet in-channel SIG’s a bit better. They could go slightly more deaf when a crystal filter was fitted, but that could be compensated for and where the filter was properly fitted and rig aligned, the actual effect of the added insert loss of the crystal filter got offset by clearer in channel reception characteristics.

So aside from the stuff about the antenna setup, take the rest as insight and use as needed. Oh, a lot of cheap SWR units are as reliable in their indication as a chocolate teapot is usable for making tea with. The one I used was one of a pair that were commercial grade items I got and refurbished when they got replaced in service so I got their calibration verified post refurbished where I obtained them from. Like any piece of test gear, not all are equal and unless you’ve somehow got working verified calibrated items exreceive, treat readings with a huge ogre handed pinch of salt at best. RF test gear is an area where there are few examples of quality retailed at bargain bucket prices.

So I guess I can just close this and simply say test and evaluate - just keep what I’ve mentioned in mind as you assess and setup with a good SWR and provided you’d be within your license still, should be good to go. Set up correctly, as they say, any antenna is better than a few feet of wet string :slight_smile:

You can use that antenna for your radio, no problem.

The only real issue with physically big vehicle antennas is bashing them, and the huge forces they can exert on whatever you attach them to. If you can fit them securely, go for it. On my van,I’ve gone back to shorter and less good antennas because the trees in my road are too ■■■■ low!