Training & Nutrition - getting heart rate up?

Hi,
I am trying to keep my cadence above 90 and my heart rate in the 65%-75% of max on my training spins to build a good base and lose some additional weight.
Where I live is relatively flat with small undulating hills.
I am finding my heart rate drops below the 65% on the flat/slight down hill sections. How do I get it back up again? Is it spin in higher cadence or push bigger gears? I know there will be points on the spins where it will go down but I want to minimise these. I would like to keep it above 65% for the duration of the spin while keeping a good high cadence.

All advice appreciated
Thanks

I am finding my heart rate drops below the 65% on the flat/slight down hill sections. How do I get it back up again?
Pedal harder.

Riding in the "fat burning zone" burns less fat per hour than riding faster. It's advantage is that you can do it for longer, thus burning more calories per ride. But unless you are doing longer rides to take advantage of it, it's not burning extra fat vs going faster.

If you used a formula to calculate your max HR, it's most likely wrong.

As you continue riding it will be natural that you will have to push harder gears to get the same workout. I used to try and stay in zones 2 and 3 for fat burning benefits. But it was so boring. I have been pushing harder and trying to raise my max heart rate. Technically my max HR is supposed to be 162, I am 58. I can now hit 170 and hold that for @5 minutes. My blood pressure is better, fat it being burned and I have more energy. If you have a couple months in building your base up I would consider using half your rides as Interval and strength building rides.

1) Start an interval program
2) Aim to improve your average MPH
3) Aim to improve your speed over whatever hills you have

I try to ride at a certain heart rate which happens to be 170. If I start dipping below that and I'm already spinning at 90, then I drop a gear and push that one to 90 RPM, and so on until I'm where I want to be heart rate-wise. This does, though, assume you know your max heart rate (and correspondingly, your zones). It's a simple way of going about it.

To answer your question, if you HR drops, grab another gear and spin. Pushing harder will get you knee pain muy rapido.

I am finding my heart rate drops below the 65% on the flat/slight down hill sections. How do I get it back up again? Is it spin in higher cadence or push bigger gears?In general the rapid movement of limbs will develop a higher rate than greater resistance. So for the situation you describe "spinning out" could work.

In any case, I think your question reflects misplaced emphasis on HR. You can still have targeted HR-based workouts in hilly areas even if you inadvertently have to "rest" during a descent. One way is to think in terms of "time-splits" - and the average HR for each period.

Shift, baby, shift. You regulate your HR and power output with the paddles on your brifter. You adapt your cadence to what feels best for you over a goodly distance. Don't climb at the same cadence you use on the flat. Go 10-15 rpm lower. You're just not shifting enough. Of course I'm assuming you're not riding a fixie or a SS bike. If you are riding a one-gear bike, well that's why people ride them, to vary the workout more.