VHF/UHF ANTENNA vs UHF ANTENNA

[N4KVE]

For years I have been using dual band antennas with my VHF/UHF radios. I have been happy with the Larsen 2/70 cm antenna, the Diamond NR770BMNO antenna & a Diamond SG7200NMO antenna. The Larson claims 3.5 DB on UHF, while the Jap antennas claim 5.5 DB, yet all 3 seem to work the same. I need the max gain to work a few repeaters 2 counties south of me. My radio is a 100 watt MCS2000, & some days are better than others. My question is does the Larson' 3.5 DB = the Diamond's 5.5 DB? Larson shows a NMO440C which is a UHF only antenna, but also rated at 3.5 DB. So my question is since I only use UHF in the car now, should I go to the UHF only antenna, or would the dualband antenna work just as well on UHF? Is the dualband antenna a compromise to work on both bands? Is there a better UHF antenna I could use? GARY N4KVE

 

[gewecke]

Put simply, a separate tuned antenna for each band operated is almost always the most efficient combo, but obviously shortcuts sometimes have to be taken.

73,
n9zas

 

[fineshot1]

Gary - I experimented on this very subject many a year ago and
between the Larsen 2/70 vs a UHF gain ant tuned to 440 and could
not notice enough of a difference. Save yourself the hassle and stick
with the Larsen 2/70.

 

[n5ims]

Them both having the same gain factor is probably correct. One is listed as having higher gain since it's being reported in dBi while the other is reported in dBd (there's a bit more than 2 dB difference between the two reporting methods. Somewhat like reporting that a certain car gets 30 Miles Per Gallon while another (that gets nearly exactly the same gas milage) gets a whoping 113.5 Liters Per Gallon.

Always assume that gain is reported in dBi unless it specifically states dBd and when comparing reported gain values add 2.1 dB to any antenna reported to be in dBd to keep things fair. The marketing department likes to use the bigger numbers since they sound, well bigger. Don't fall for their tricks!

To answer your second question, either the 2/70 or 440C should work well for you (I have both). They perform the same on UHF as close as I can tell without using any special equipment. The UHF only antenna is about 6" shorter (mostly the bottom loading coil, which is there for VHF on the 2/70) so if you need something just a tad shorter you may want to go with the UHF only antenna, but otherwise it's pretty much a wash.

 

[N4KVE]

Thanks for the help. Yes the Larsen antennas are rated in dBd, while the Jap antennas are rated in dBi, so actually the gain figures are equal. But one more question. For the last few years I have been using the Jap antennas which are rated at 200 watts. The Larsen is rated at 100 watts. I have been using the 100 watt MCS2000 with the Diamond antennas since they are rated higher. So would it be safe to use the Larsen antenna which is rated at 100 watts with a radio rated at 100 watts, or should I go with the Larsen NMO440C which is rated at 200 watts? GARY N4KVE

 

[LtDoc]

That's another one of those 'complicated' questions, lots of variables, and there's no absolutely sure answer to it. If you have the option, it's safer to use an antenna rated for more than you'll ever put into it. It depends a lot on how 'optimistic' the manufacturer is, you know? One of the determining factors is how well the stinker is tuned, if it's resonant and just how 'resonant' is it? That's always an individual thingys.
- 'Doc

(More is better till it get's ridiculou$. It can get ridiculous very quickly.)

 

[N4KVE]

Well I ordered the Larsen 440C from Universal Radio. It was $6 less than AES, so for $34 including shipping, I decided to take a chance. I'll report how it does when it gets here. GARY N4KVE

 

[N4KVE]

All 4 antennas seem to work about the same, which is very well. I'm just trying to help receive the repeaters that are 60 miles away. GARY N4KVE

 

[N4KVE]

Update

Well after about 3 weeks of using the UHF vs the dual bander, it seems the 2/70 antenna works slightly better for those repeaters 60 miles away. Up to 40 miles, there is no difference, but the 2/70 antenna does improve the reception on the Miami repeaters. Maybe because it's 8" taller, there's a height advantage, who knows, but the dual band antenna has bigger ears. GARY N4KVE

 

[kayn1n32008]

Wirelessly posted (Mozilla/5.0 (BlackBerry; U; BlackBerry 9780; en-US) AppleWebKit/534.8+ (KHTML, like Gecko) Version/6.0.0.600 Mobile Safari/534.8+)

The Larsen should be ok at 100w as long as you are moving, if it has a closed coil, be VERY cautious while sitting still. I have seen a Larsen 2/70 with a melted closed coil with 75w on 2m! The person was VERY long winded on high power and sitting still the coil could not handle the power. The open coil should be ok though.

 

[LtDoc]

The one single thing that would improve reception and you don't have to play the 'gain' game would be to get the antenna higher. That has problems too, but it's still the single one thing that can make the most improvement without changing antennas.
An antenna of a particular 'style'/'size' will typically have the same gain as any other antenna of the same 'style'/'size'. Increase the electrical size and you typically increase gain. There are limits though.
- 'Doc

 

[jparks29]

Well after about 3 weeks of using the UHF vs the dual bander, it seems the 2/70 antenna works slightly better for those repeaters 60 miles away. Up to 40 miles, there is no difference, but the 2/70 antenna does improve the reception on the Miami repeaters. Maybe because it's 8" taller, there's a height advantage, who knows, but the dual band antenna has bigger ears. GARY N4KVE

radiation pattern will come into play....

Most dual bands are 5/8 or 1/2 wave on UHF, which will squeeze the signal out a little farther, albeit at a higher angle..

 

[KB7MIB]

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I thought a 5/8 wave vertical antenna had two main lobes? One compressed down low which extended the range vs a 1/4 wave vertical antenna, and a 2nd lobe, with a higher takeoff angle, which would be good for repeaters on mountain tops, or skyscrapers?

 

[nd5y]

I thought a 5/8 wave vertical antenna had two main lobes? One compressed down low which extended the range vs a 1/4 wave vertical antenna, and a 2nd lobe, with a higher takeoff angle, which would be good for repeaters on mountain tops, or skyscrapers?

No, that is a 3/4 wave. The minor lobes on a 5/8 (.64) wave are a lot smaller, like the side lobes on most yagis.

 

[LtDoc]

To a large extent the radiation pattern is a function of antenna length and location. The same antenna can have a number of patterns depending on where/how the thing is mounted. The patterns usually shown for a particular antenna are only a 'general' thing, never absolutely the same in all instances. If you use one of the antenna modeling programs and play with the mounting height, the radiation pattern can change shape quite a bit.
- 'Doc

 

[KB7MIB]

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How common are mobile 3/4 wave antennas? I've only seen 1/4 wave, 5/8 wave or collinear antennas for mobile use.

 

[nd5y]

Nobody makes 3/4 wave antennas.
A 1/4 wave antenna will work on the 3rd harmonic of whatever frequency it is cut for.
For example a 19" long 2m 1/4 wave antenna will function as a 3/4 wave on 70cm. (3x146=438)
The SWR will probably be higher and it will have the high angle lobes that are usually not desirable.