Alec

@WRYZ926 Great information here. Thanks!! I was aware that the different ground plane clearances of the various conductive sections of my truck (Hood, Cab & Bed) would alter the reflective properties of the RF counterpoise. I was not aware that the vertical distance between the antenna on the cab and my truck bed, although more than a wavelength at GMRS frequencies, would effectively nullify all effectiveness of my truck bed as a ground plane!! I'll certainly need to research and read up on that... My current magnet mount has been placed very near the 3rd tail-light location and suffers from the same ground plane issues and so far the performance has been acceptable to me. I currently use a 5/8 wave GMRS antenna (although I'm not married to it - on rare occasions I swap it out for my 2M/70cm antenna) and at GMRS frequencies only about 15" of surface area is needed for an effective ground plane. So, if RF optimization were my ONLY consideration, shifting that GMRS antenna forward a few inches from the rear of the cab would probably be close to as good as it is going to get for my vehicle... Many of the comments thus far suggest just biting the bullet and drilling that 5/8" hole in the roof of the cab of my truck and installing an NMO mount as the best approach. I really do appreciate the feedback and comments! But OUCH... I just have a real aversion to that, and most likely will never do so and would rather just live with a magnet mount. Perhaps I should have mentioned in my OP that my truck has a retracting moon roof which significantly complicates things as far as drilling and mounting anything in the roof of the cab more than a few inches forward of that 3rd tail-light because of the sliding glass, retraction motor, etc. of the moon roof inside the roof of the cab. And yes, the moon roof glass too impacts my ground plane to some degree. If it's not one thing, it's another... But still I'm seeking the convenience of some kind of permanent mount, even if not optimal from an RF perspective, which as stated in the OP, is why I'm looking at whatever alternate mounting solutions I can find. Hood/fender mounts are often a no-drill option but those too often force you to route the coax thru an RF noisy engine compartment. They also tend to force you to mount them on either the right or left side of the vehicle which alters the radiating pattern to the sides, not to mention that you usually end up with the antenna being lower than the cab of the truck thus causing a certain amount of unwanted RF reflection if not some signal blockage. I conceded that there is no perfect solution, and things are never as simple as they may sound. So, while I'm not yet SOLD on this 3rd tail-light mount, and it is very expensive, thus far it seems to be a path to solve my specific problem. This is why I'm looking for folks with first-hand experience specifically with these 3rd tail-light types of mounts, even though any and all feedback is much appreciated! Thanks again...

@WRUE951 Thanks for sharing your experience. I hear you. I'm not thrilled about pulling the headliner either, as those things always turn out to be more difficult than we anticipate, but I figured that if I drilled a hole for a mount in the roof of the cab, I'd be going down that path anyway. I agree the price is very steep for an antenna mount! For my make/model/year truck the mount is no bargain @ $360 but if it avoids the potential for water leaks that can occur when you drill holes in the roof, (I've seen a lot of that with fleet vehicles) it is, from my perspective, worth the price. Also, I'll be sure to check Ebay "IF" I end up going with this type of solution. @Socalgmrs Thanks for your comment. On my 2018 GMC Sierra pickup truck (w/ a 6' truck bed) the 3rd brake light is very close to being as centered, front-to-back and right-to-left, as you can get. Thus, I don't believe this location sacrifices 180 degrees of ground plane as you suggest. I'm not wanting to direct the radiating pattern in any specific direction. If I'm not understanding your comment here, please explain... Thanks

Greetings... Does anyone out there in GMRS land have any experience with 3rd brake light antenna mounts? Like these: https://bulletproofdiesel.com/collections/antenna-mounts Do they leak? Are they strong enough for a ~32" antenna? I've been using a magnet mount on my GMC truck, running the coax out thru the rear sliding window, and placing the antenna on the roof of the cab just in front on the 3rd brake light. From an RF signal perspective, it works just fine, but I would not mind transitioning to a more permanent antenna mount. I like the idea of a no-drill solution and keeping the antenna's location more-or-less in the center of the ground plane. That makes this antenna mounting option seem like an elegant solution even though it is a bit expensive. Any feedback or personal experience is appreciated... ** UPDATED INFO: my truck has a retracting sun/moon roof which significantly complicates things as far as drilling and mounting anything in the roof of the cab more than a few inches forward of that 3rd tail-light because of the sliding glass, retraction motor, etc. of the sun/moon roof inside the roof of the cab. Besides, I really don't want to drill into the roof of my cab!!! Thanks

Hi Frank, I can understand your frustration but try to not get discouraged and stay excited. Things will get resolved. A random consideration: In many areas, depending upon the number of GMRS / FRS users in the area, communicating via simplex can be a real challenge. The 462Mhz/467Mhz frequencies used for GMRS channels are typically only capable of line-of-sight communications. Factor in buildings, trees, foliage, terrain and other things that tend to absorb signals in that frequency range and distance for 2 respective ground stations becomes quite limited. Obviously, this is why so many GMRS users tend to focus a lot on mountain/tower/building top repeaters. But in terms of getting an accurate signal check and test of your equipment, you are, IMHO, probably better off using simplex because when you do, people will be hearing your radio signal directly and not the signal of some presumably higher-powered, elevated repeater. If you can hear people talking on the non-repeater channels 1 thru 14 then you are almost certainly hearing simplex radio transmissions and chances may be good that if you are in range to hear them, they would be in range to hear you! If you hear someone on a simplex channel and attempt to communicate with them but do not get a response (assuming they are not just ignoring you!) the issue may be the other operators are using CTCSS or DCS sub-tones. IOW, it may not be your radio that is the issue, it may be the other person's radio that is the issue. The use of these sub-tones or "privacy codes" as they are often improperly called, restrict the user's radio to only hearing (breaking squelch) when that specific matching sub-tone is detected by the receiving radio. This creates the illusion that the conversation is private because unless the transmitting radio is sending out the correct matching sub-tone the receiving radio won't open squelch and play any audio. The majority of repeaters require a specific CTCSS or DCS sub-tone. So, your radio may be transmitting just fine in terms of distance, but the other GMRS users are not hearing you because of a specific setting on their respective radio. While the use of CTCSS or DCS codes on simplex is routinely discouraged many people do use them because they think it makes their conversation private. Additionally, some radios ship from the factory with CTCSS or DCS codes pre-programmed, so people just take the radios out of the box and start using them having no idea that these sub-tone codes are in use. So don't get discouraged too quickly. Some GMRS radios or scanners have the ability to display if a CTCSS or DCS tone is being transmitted by another radio and if you can detect this, it may allow you to set your radio to use the same sub-tone and make it "heard" by others. Doing tests of your mobile equipment with another handheld radio and a friend where you control the settings is the easiest path to take. If you need to rely on the assistance of other random people, then you must consider how the other person's radio might be configured. Good Luck! BTW, there is nothing wrong with talking to random people via GMRS! A random person may turn out to be a friend you just haven't met yet.

Hi Paul, Many radios on the market, including the Tidradio H8, ( I own one too) have the ability to receive on frequencies or bands that they do not have the ability to transmit on. This is why you were able to program frequencies into the radio that you can only receive and not transmit and it allows the device to be used as a frequency or channel scanner. At the risk of stating things that you may already know, here is a bit more explanation. The transmit frequency range(s) are locked by the manufacturer in the firmware or software of the radio. This is most often done so that the radio remains compliant with the certification for that device which has been granted by the regulatory agency for your country under their respective rules. I am not familiar with the regulatory entity in Australia so I cannot speak to that entity or their rules, but for example here in the USA frequency allocations and things such bandwidth, as how much transmitting power may be used etc. are defined and regulated by the Federal Communications Commission, aka the "FCC" and often radios sold here in the USA are FCC certified for use in a specific band that is defined by a "part" or section of the FCC rules code. For example, a GMRS radio here in the USA would be authorized under part 95 of the US FCC code and in order for the device to be Part 95 compliant or certified the radio would be required to adhere to certain frequency restrictions, band widths and power levels, etc. In many cases the radio might be capable of transmitting on other frequencies or bands or use more power etc., but these software imposed restrictions prevent that from happening so that the device can be compliant with those rules associated with specific uses. Some devices can be "Unlocked" to remove these artificial, software imposed, restrictions. You mentioned that you were considering getting your HAM radio license. In the case of the Tidradio H8, if you purchased the HAM version of the radio and not the GMRS version, that specific device does have the ability to be unlocked (Google it). However, while unlocking the device may allow it to transmit on the HAM frequencies and GMRS frequencies (or as you referred to the CB frequencies) that does not mean that the device is certified or authorized for such use by your communications regulatory authority, and you may or may not be licensed to use all the frequencies that the device is capable of working with, so be sure to do your homework about your local rules and requirements.

I agree with the previous posts that the most probable culprit is that the RT15 radios are using CTCSS or DCS Codes (see page 6 of your RT15 manual - I found it on the Retevis website here: Retevis RT15 Mini Lightweight FRS for Business Radio, usb charging, license free walkie talkie). Page 1 of that RT15 manual shows a diagram of the radio and according to that diagram the radio has a "Monitor" button on the side just below the PTT key. If you long press that button, it should, temporarily bypass any pre-programmed CTCSS/DCS tone code squelch. So, if you have all radios on the same channel (avoid channels 8-14 as your base station probably will not transmit on those channels) then long press that Monitor button on the RT15 while you transmit on the Midland GMRS base station and you hear the base station radio transmission come thru the speaker on the RT15 you will have pretty much confirmed that the issue is that your RT15 FRS radios have factory pre-programmed CTCSS or DCS Codes enabled. To solve this issue, you will need to reprogram these RT15 FRS radios to remove the use of CTCSS/DCS tone codes. As has been suggested by @AdmiralCochrane, you could also program your Midland 105 base radio to use the same CTCSS/DCS codes as the RT15 are using, but personally, I would not recommend that approach as doing so will limit your ability to communicate with others. Hope that helps.

Sorry, I should have been more specific. It's a 2018 GMC Sierra

From what I've observed magnet mounts seem to be rated in terms of "pounds of pull strength" which appears to be a rating of how many pounds of pressure or pull is needed to remove the magnet. Again, just casual observations on the various magnet mounts offered for sale via Amazon suggests that the larger the magnet mount diameter the more pull strength it tends to have. I would guess that getting a larger diameter magnet mount or one with more pull strength (or a smaller antenna with significantly less wind resistance) might allow you to overcome this issue of the mount slipping. FWIW, I've a GMC Sierra and use a magnet mount with a 32" whip, with no issues. But hey, I'm not a certified magnetologist, so your results may vary.