I am about to move and put an antenna in the attic of the new house.  I understand this is less desirable than mounting it outside but that is where it will be.  My plan is to mount a dual band antenna in the attic and drop LMR 400 to the basement where the radio will be.  The radio is a FTM-400 DX.  My question is will a 10’ antenna or a 6’ antenna be better?  The top of the antenna will be the same height the only difference will be the base height above ground.  Is there any advantage to the shorter antenna mounted higher above ground at the base?  

What kind of antenna are you talking about?
Vertical, dipole, yagi?  Hard to answer your question without some more info.

This, very important.

I'd assume you're asking about two models of commercial dual band vertical base antennas, and in that case it's typically better to get the longest one you can manage size wise, as they are usually colinear designs and longer means more stacked elements in the antenna, which means more line-of-sight gain.

Big is better but beware in that location bigger may mean more chances for some nearby metal to de-tune your antenna.

In your situation I'd keep it as small and simple as possible in order to figure out what works first. For example, if it's in an attic the repeaters you want to hit might be better reached in a different location than immediately above your basement shack.

It will be a vertical since this is for FM VHF UHF.  Assuming the same antenna build just a different size ( EG Comet GP -3 vs GP-6) is there an advantage to the longer antenna if it is not physically higher and the base is lower to the ground bases on the max height imposed by the roof?

The house is on a bluff that stands ~200’ above the surrounding landscape.  There are other houses up on this bluff as well but it’s a single street so a home across the street from the front and nothing in the back.  

There is little in the attic area. The home’s electrical wires for lighting and outlets do run across the bottom of the attic but no ductwork etc in this attic.  It’s a large relatively open area with about 15’ of height above the floor to the roof peak.  

My real question though is whether a longer antenna with higher gain is of any value when the overall height is capped at a set height.

I do expect this antenna to be an improvement of the current set up which is a N9TAX roll up Jpole on the wall next to the radio desk.

More antenna gain is always better. Get the big one.

Yep, more gain is always better. There is some question whether something like a Ventenna would do a better job because it's outside the house and a bit higher, not to mention almost unnoticeable.
73,
Rob

+1

get the taller one, and keep it away from other objects, heating ducts, etc.

Not always.

Look at the pattern coming off of the antenna.

For instances where you want a large elevation bandwidth that is true. But this isn't one of those cases. He's not setting up for satcom.

Since you're on a 200' bluff with a 15' clear attic, go with the longer antenna. From what you described, the house wiring will be beneath the antenna's ground plane, so it shouldn't have too much of an effect.
But to confuse the situation, since you're on the 200' bluff, depending on the distance you intend to communicate, there may not be a whole lot of noticible difference between the two antennas.

You may have to reduce your power because of interference with the various home systems.

Umm, your roof is a non-conductive material, right?

Thanks for the recent replies. Haven’t checked the thread in over a week since we have been busy moving.  

Yes the electric runs for the house will be below the radials for the antenna by about 5’  with the longer antenna.  

The roofing material is architectural shingles, roofing felt, and plywood. So I would expect minimal interference.

No it's not.

It's a known fault of the amateur market dual band antennas that the longer ones have pattern skewing. They are series fed from the bottom, so the pattern skews up or down from the horizon as you change frequency. Since most of them are designed in Region 1/3 markets they're already mis tuned in the US, which just makes the problem that much more noticeable. It's very noticeable on 440, even on 2m some of the time. More gain pointed in a useless direction means less gain where it it useful.

I've seen and used a bunch of both and strongly suggest staying with the 8 or 10 foot types (or smaller) instead of the big 17/18 foot ones.

I suggest a Diamond X30 or something similar for vhf/uhf, it's 4.5'.

Funny things happen with you put a hat on a vertical antenna so I'd go with the x30 so you can keep the top of the antenna away from the ceiling.  You'll get excellent performance from the X30, and it's a manageable footprint.  I might experiment with having it at different heights, and be prepared for weirdness as it gets closer to the ceiling like unusual signal degradation.  There's a reason FM stands for fn magic.