2M Antenna Recommendations?
- Thread starter Dave Bennett
- Start date
DaveInDenver
Middle Income Semi-Redneck
Laird is legit, but to call Larsen cheap isn't right. Larsen antennas hold up in commercial environments just fine for years, too. There is reason to build a simple spring contact as they do as well, it's less likely to be affected by dirt, temperature and moisture. I've had center conductors like Laird uses (although not Laird FWIW, the do in fact use a substantial spring) get sticky and not make contact at all being stuck in a retracted position. My $0.02 is the quality of the interface is as important as the design.
Also there is a lot more movement in the NMO interfaces than you'd think, particularly with thin sheet metal and large antennas, so if that center pin conductor loses it's freedom of movement regardless of how its constructed you can start seeing issues, so it's important to inspect it once in a while and definitely any time you might remove the whip.
Also there is a lot more movement in the NMO interfaces than you'd think, particularly with thin sheet metal and large antennas, so if that center pin conductor loses it's freedom of movement regardless of how its constructed you can start seeing issues, so it's important to inspect it once in a while and definitely any time you might remove the whip.
You may be right. Admittedly, I have little experience with this stuff. I can see building things simply to eliminate potential failures but my new antenna sure feels stout. Like I said, the spring is so stiff it's actually a little bit of work to install it. I keep a cap on my NMO mount when not in use and I store the antenna inside the car. Barring a bath in mud water or something similar, I think it would take a lot to make the contact stick and cause connection issues but anything is possible.
For what it's worth, I'm not trying to put anyone's antenna down. These are only my observations. Also, mine was not a Larsen (At least I don't think it is... has "A S" printed on it).
DaveInDenver
Middle Income Semi-Redneck
Antenna Specialists probably, which merged and became part of PCTEL along with Maxrad.
taugust
Adventurer
As I'm reading this thread, my first thought was that the high SWR was caused by the center contact. Then the OP shows the contact, and there we go. The end of the spring tab should be at about a 45* angle upward in that view to ensure good contact. I agree with DaveInDenver that these antennas need to be inspected periodically, as the center contact will occasionally move away. I have several of the Larson antennas and that seems to be common in use. Also, spray a little WD-40 inside to prevent moisture and corrosion inside. I have had them corrode as well.
I run this flexi-whip cut to 19.6". It's quite flexible and not an issue with low hanging trees or entering the garage. I run mine on an NMO mount. It's by no means a high gain antenna, but it works quite well. I ordered it from Tessco.
http://www.sti-co.com/antenna-products/public-safety-antennas/flexi-whip
http://www.sti-co.com/antenna-products/public-safety-antennas/flexi-whip
DaveInDenver
Middle Income Semi-Redneck
Been doing some characterization of the STI-CO as a dual band antenna since a fair number of people have figured out that it is an off highway friendly whip.
People may not realize that a 1/4λ 2m antenna will be 3/4λ on 70cm and the amateur spectrum is aligned such that the phone part of our 70cm band is reasonably close to an exact 3rd multiple of our 2m spectrum (146MHz x 3 = 438MHz). Bottom line, if you have enough bandwidth on 2m & 70cm you can usually get a decent electrical match on both. This will only work with 1/4λ whips, not 1/2λ or 5/8λ because those must use feedpoint networks to get your 50Ω match.
These are modeled SWR charts for a 19-1/2" whip over perfect ground.
So you can see the match is reasonable as you would expect. Remember that a 1/4λ or any multiple thereby will present a 37Ω load at the feedpoint, so this is textbook antenna 101 from your tech exams.
This is not the problem, though. Since not everything can be perfect, a 3/4λ antenna has a unique radiation pattern. The null at 22° will be around -16dB right in the meat of the 70cm band. This is problematic because that's roughly where you'd expect to have an aspect to repeaters when you consider their position relative to your vehicle most of the time.
To give a better grip on what this means, -16dB would be about a 40 times difference. Put another way, if you send 100W into the antenna the receiving antenna is seeing a signal that would be the same as a 2.5W antenna at unity gain or 0dB. IOW a person with an 1/4λ on top of an HT will have the same chance of getting into a repeater as you would with this antenna at 100W, at 22° anyway. The null is fairly narrow, so it would take just a few degrees of aspect change to change that example, but this does suggest that a flexible antenna like the STI-CO will fade as you drive down the road and that is in fact what irritated me the most, that the signal strength would constantly bounce between S9 full quieting and S2 barely keeping the squelch open.
This is actual measurements from whips of a couple of lengths. Notice the parasitic effect of the PL259/SO239 and probably the coax length and RG-58 itself. These start to become important on UHF. I really do wish my duplexer had N connector, but it is what it is.
So this solution is not an ideal dual band but might be a decent compromise for trail duty. I use 70cm often in town for rag chews, more so now with the increasing use of DMR radios which are almost always commercial UHF units. So I went with a dual antenna system with a duplexer fanning to two whips, each cut for 1/4λ.
People may not realize that a 1/4λ 2m antenna will be 3/4λ on 70cm and the amateur spectrum is aligned such that the phone part of our 70cm band is reasonably close to an exact 3rd multiple of our 2m spectrum (146MHz x 3 = 438MHz). Bottom line, if you have enough bandwidth on 2m & 70cm you can usually get a decent electrical match on both. This will only work with 1/4λ whips, not 1/2λ or 5/8λ because those must use feedpoint networks to get your 50Ω match.
These are modeled SWR charts for a 19-1/2" whip over perfect ground.
So you can see the match is reasonable as you would expect. Remember that a 1/4λ or any multiple thereby will present a 37Ω load at the feedpoint, so this is textbook antenna 101 from your tech exams.
This is not the problem, though. Since not everything can be perfect, a 3/4λ antenna has a unique radiation pattern. The null at 22° will be around -16dB right in the meat of the 70cm band. This is problematic because that's roughly where you'd expect to have an aspect to repeaters when you consider their position relative to your vehicle most of the time.
To give a better grip on what this means, -16dB would be about a 40 times difference. Put another way, if you send 100W into the antenna the receiving antenna is seeing a signal that would be the same as a 2.5W antenna at unity gain or 0dB. IOW a person with an 1/4λ on top of an HT will have the same chance of getting into a repeater as you would with this antenna at 100W, at 22° anyway. The null is fairly narrow, so it would take just a few degrees of aspect change to change that example, but this does suggest that a flexible antenna like the STI-CO will fade as you drive down the road and that is in fact what irritated me the most, that the signal strength would constantly bounce between S9 full quieting and S2 barely keeping the squelch open.
This is actual measurements from whips of a couple of lengths. Notice the parasitic effect of the PL259/SO239 and probably the coax length and RG-58 itself. These start to become important on UHF. I really do wish my duplexer had N connector, but it is what it is.
So this solution is not an ideal dual band but might be a decent compromise for trail duty. I use 70cm often in town for rag chews, more so now with the increasing use of DMR radios which are almost always commercial UHF units. So I went with a dual antenna system with a duplexer fanning to two whips, each cut for 1/4λ.
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DaveInDenver
Middle Income Semi-Redneck
BTW, the standard 1/4λ elevation pattern. So you can see that a 3/4λ antenna has a significant gain advantage between 30° and 60° with that huge lobe at 45~55° and is a bit better at the horizon (notice the normalized gains).
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DaveInDenver
Middle Income Semi-Redneck
Essentially just verification of the 3/4λ antennas. Would it be helpful to explain impedance and radiation characteristics such as azimuth and elevation patterns? The problem with 3/4λ antennas is they have what amounts to a large region where they are deaf and don't work well. That's the null in the elevation pattern. The radio won't mind being attached to it at all, in fact the antenna more or less looks electrically the same on 2m and 70cm.
But the real world coverage between the two bands is significantly different. Close in, like amongst a trail run, it won't matter much. At very long distances at the horizon it will cover alright. It's the intermediate ranges, like is typical of repeaters just slightly above the horizon, where the 3/4λ antenna will experience a lot of signal strength fade.
But the real world coverage between the two bands is significantly different. Close in, like amongst a trail run, it won't matter much. At very long distances at the horizon it will cover alright. It's the intermediate ranges, like is typical of repeaters just slightly above the horizon, where the 3/4λ antenna will experience a lot of signal strength fade.
Uncle Beer
Observer
I run a comment that I'm very happy with. Compatible to the nr770 from diamond. But I just noticed that comet has a new flex model on an nmo base with even high gains.
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